Question everything electrical. (1 Viewer)

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Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
I have recently acquired a pair of mastervolt inverters and a charger, second hand from someone who used them for an off grid workshop. They have nice fat wires with them.
I'd like to install one but even if I get someone qualified to install it, it would be nice to have a rudimentary understanding of how they work and what they do. Oh and what kit is really needed for them to work properly.

So please bear with and have some grace for the electrically illiterate.

batteries seem to be complicated, the holy grail being lithium, a level of holiness I one day aspire to. I have 2x mastervolt 100 ah batteries which seem to be nackered (they were a freebie with the kit above). numerous recovery cycles on the Noco genius haven't managed to get one above 10v under load and the other above flatline.

I also have an old car battery, 85 AH which is stop start compatible. Would this have the punch to run the inverter, which is a 1200w, powering an 800w microwave?
the blurb on the inverter suggests it could handle it, but reading several hundred threads about inverters and batteries suggests that the batt would not be able to keep up with the draw for any amount of time. (likely to be a couple of minutes reheating food). In my mind (remember it's dyslectric) I have the idea that I can have a separate system with the charger to the battery from the existing solar, and then a separate 3 pin socket to power the M/wave or toaster. this doesn't then have to be plumbed into the rest of the EHU system on the moho and I can just switch the inverter on and off when I want to use it.

What is a busbar?
What is bonded?
Do I need an inline fuse or not?
Is it worth getting the inverter wired into the EHU system?
MPPTs and RCDs are a mystery to me.


I've taken pics of what is currently there, to aid understanding but if you are kind enough to explain anything, please don't use any technical terms or jargon without explaining it. The last time I learned anything about electricity I was at high school and was certainly more interested in anything else than what the teacher was talking about!



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6ytqm5kn.png
 
OP
OP
S
Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
I have recently acquired a pair of mastervolt inverters and a charger, second hand from someone who used them for an off grid workshop. They have nice fat wires with them.
I'd like to install one but even if I get someone qualified to install it, it would be nice to have a rudimentary understanding of how they work and what they do. Oh and what kit is really needed for them to work properly.

So please bear with and have some grace for the electrically illiterate.

batteries seem to be complicated, the holy grail being lithium, a level of holiness I one day aspire to. I have 2x mastervolt 100 ah batteries which seem to be nackered (they were a freebie with the kit above). numerous recovery cycles on the Noco genius haven't managed to get one above 10v under load and the other above flatline.

I also have an old car battery, 85 AH which is stop start compatible. Would this have the punch to run the inverter, which is a 1200w, powering an 800w microwave?
the blurb on the inverter suggests it could handle it, but reading several hundred threads about inverters and batteries suggests that the batt would not be able to keep up with the draw for any amount of time. (likely to be a couple of minutes reheating food). In my mind (remember it's dyslectric) I have the idea that I can have a separate system with the charger to the battery from the existing solar, and then a separate 3 pin socket to power the M/wave or toaster. this doesn't then have to be plumbed into the rest of the EHU system on the moho and I can just switch the inverter on and off when I want to use it.

What is a busbar?
What is bonded?
Do I need an inline fuse or not?
Is it worth getting the inverter wired into the EHU system?
MPPTs and RCDs are a mystery to me.


I've taken pics of what is currently there, to aid understanding but if you are kind enough to explain anything, please don't use any technical terms or jargon without explaining it. The last time I learned anything about electricity I was at high school and was certainly more interested in anything else than what the teacher was talking about!



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View attachment 910809
I have the above kit installed. Not really sure what most of it is doing.
 
Jul 12, 2013
3,915
5,344
The City of Henlow
Funster No
26,906
MH
Adria Supreme
Exp
Since 1980
A bus bar is a strip of metal with either a negative or positive cable attached to it and several connecting points to allow additional connections to be added
 
Jan 2, 2024
711
940
Lincolnshire, UK
Funster No
100,498
MH
Peugeot boxer
Exp
2020
Your 85 ah car battery is unlikely to run the inverter with a 1200w load for long if at all.
1200w at 240v means the inverter will be drawing 100 ah plus inefficiency 110 ah, also inverters usually have a low voltage shut down..so when you demand 110a from your battery the voltage will drop and the inverter will shut down.. probably.
You can try it on the bench wire it to your fully charged battery with a couple of as short as possible heavy battery leads plug a small load(table lamp in if it works try your microwave..if the microwave is digital the inverter may need to be pure sine wave not modified sine wave...it should say on it
 

MisterB

LIFE MEMBER
Feb 25, 2018
6,440
14,701
Essex
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52,564
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Adria 670 SLT
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enough to know i shouldnt touch things i know nothing about ....
A busbar can also just be the connecting 'bit' between two batteries or cells....

Can I suggest you watch a few you tube videos on camper electrical installations, Greg Virgoe does a good one, and I'm sure other Funsters could recommend other videos to watch and learn from as there are lots out there. You will then have a better knowledge of the components that make up an electrical system. It's not difficult once you understand what does what and where it connects to and from.
If you're determined to upgrade your electrics, it doesn't all have to be done in one go but I would always suggest oversizing your cables as you progress, so you don't need to keep replacing them every time you upgrade a particular piece of kit and if you're running cables through voids etc, always try to think ahead and run additional cables for future projects, even if you never get round to doing them. It can save lots of hours spent taking things apart and trying to put them back together again. Lots of motorhome builders install wires 'just in case' ..... and lastly ALWAYS label any additional cables at both ends !!

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Apr 9, 2022
469
470
Funster No
87,949
MH
Cathargo
Exp
Newbie
Mastervolt kit is very nice kit (certainly up there with Victron for quality) - which inverters and charger do you have? You may find that to change the settings you will need an interface to use a USB cable (printer type) and a laptop and app. As oldiesontour has said your photographed batteries will not support a powerful inverter - but at least Mastervolt stuff is pure sine wave output.
 
Apr 27, 2016
7,103
8,367
Manchester
Funster No
42,762
MH
A class Hymer
Exp
Since the 80s
First, the difference between 12V and 240V electrics. I'm sure you appreciate that electricity is the flow of something. In fact, electric current is the flow of electric charge. The chemical process inside the battery gives energy to the electric charge. The current flow carries power to where it is used. The current flow is measured in amps. The amount of power each amp can carry is called the volts.

So each amp from a 12 volt battery carries 12 watts. However each amp from a 240V mains plug can carry 240 watts.

So 960 watts from a 240V supply would need 960 / 240 = 4 amps. But 960 watts from a 12V battery would require 960 / 12 = 80 amps. This is usually summarised in the equation watts = volts x amps.

An inverter changes 12V DC power to 240V AC power. The power input and the power output are about equal. So if you want 960W output, that would be 4A at 240V. The input would also have to be 960W, and that would be 80A at 12V.

In fact, there would be a percentage loss of power of about 5% due to inefficiency, so to get 960W out, you would need to put in about 1010W, ie about 84A at 12V.

Next look at how much power the microwave requires. The '800W' on the label will be the output power, that goes into the cooking. The input power will be more than that, because a microwave is not very efficient. 800W output will probably require about 1000W input. The numbers will be there somewhere, probably in very small letters on the label underneath.

Now about batteries. To produce the power required by a microwave, the amps input into the inverter is likely to be about 84A. Your 85Ah stop start battery could produce that sort of amps, but only for the sort of time that a starter motor runs for - a few seconds maybe. Taking that amount of amps for several minutes would definitely reduce the life of the battery drastically.

To provide long discharges of 85A or so, you would need lead-acid batteries totalling about 5 x 85 = 425Ah. However a microwave typically has short discharges of a few minutes occasionally, you could probably get away with half of that, say two 110Ah batteries, and that's what many people do. The upshot is, you'll need about three 85Ah batteries to power an inverter to run an 800W microwave. Or a couple of 110Ah batteries.

One of the reasons lithium batteries are so popular is they can be discharged at a higher rate than the same Ah capacity lead-acid battery. A 100Ah lead-acid battery should be restricted to about 20A discharge, but many 100Ah lithium batteries can give 100A discharge without reducing the battery longevity.
 
OP
OP
S
Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
Your 85 ah car battery is unlikely to run the inverter with a 1200w load for long if at all.
1200w at 240v means the inverter will be drawing 100 ah plus inefficiency 110 ah, also inverters usually have a low voltage shut down..so when you demand 110a from your battery the voltage will drop and the inverter will shut down.. probably.
You can try it on the bench wire it to your fully charged battery with a couple of as short as possible heavy battery leads plug a small load(table lamp in if it works try your microwave..if the microwave is digital the inverter may need to be pure sine wave not modified sine wave...it should say on it
Hiya, I tried it on the bech with a power tool, which was able to function, I'll need to run it a while to make sure it actually works under load rather than just momentarily and cuts out.
 
OP
OP
S
Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
A busbar can also just be the connecting 'bit' between two batteries or cells....

Can I suggest you watch a few you tube videos on camper electrical installations, Greg Virgoe does a good one, and I'm sure other Funsters could recommend other videos to watch and learn from as there are lots out there. You will then have a better knowledge of the components that make up an electrical system. It's not difficult once you understand what does what and where it connects to and from.
If you're determined to upgrade your electrics, it doesn't all have to be done in one go but I would always suggest oversizing your cables as you progress, so you don't need to keep replacing them every time you upgrade a particular piece of kit and if you're running cables through voids etc, always try to think ahead and run additional cables for future projects, even if you never get round to doing them. It can save lots of hours spent taking things apart and trying to put them back together again. Lots of motorhome builders install wires 'just in case' ..... and lastly ALWAYS label any additional cables at both ends !!
Thanks, some sensible advice there. I read every thread i could find on here, and what often happens is that the language very soon becomes quite technical without an explanation, i.e people start running calculations without explaining much about what it's about, or referencing bits of kit you might already have, or might need, without explaining what they are for. Hopefully I can find a beginners guide somewhere on youtube.
 
OP
OP
S
Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
First, the difference between 12V and 240V electrics. I'm sure you appreciate that electricity is the flow of something. In fact, electric current is the flow of electric charge. The chemical process inside the battery gives energy to the electric charge. The current flow carries power to where it is used. The current flow is measured in amps. The amount of power each amp can carry is called the volts.
Thanks for the explanations, I've replied in text at the points where I have questions. OK that bit makes sense
So each amp from a 12 volt battery carries 12 watts. However each amp from a 240V mains plug can carry 240 watts.
Aaaaah you lost me at Watts. if a 12v battery carries 12w what is the difference between a v and a w? Where to the W's come from. Are they a measurement of power? I know W is the power shown on Power tools or microwaves etc.
So 960 watts from a 240V supply would need 960 / 240 = 4 amps. But 960 watts from a 12V battery would require 960 / 12 = 80 amps. This is usually summarised in the equation watts = volts x amps.
So watts divided by volts = amps. I can understand that. If my battery is 85ah isnt that the ability to give 85 amps for an hour. So theoretically it can give the 80 amps needed? But then I've heard that lead acid batteries can't run down that much. But also then we'd only be using the microwave for a few minutes say 10% of that amp hour?
An inverter changes 12V DC power to 240V AC power. The power input and the power output are about equal. So if you want 960W output, that would be 4A at 240V. The input would also have to be 960W, and that would be 80A at 12V.

In fact, there would be a percentage loss of power of about 5% due to inefficiency, so to get 960W out, you would need to put in about 1010W, ie about 84A at 12V.

Next look at how much power the microwave requires. The '800W' on the label will be the output power, that goes into the cooking. The input power will be more than that, because a microwave is not very efficient. 800W output will probably require about 1000W input. The numbers will be there somewhere, probably in very small letters on the label underneath.
OK will take a closer look at the microwave, see pic.. I'm not going to try and interpret that. But it's a 500w microwave not an 800w as previously thought. So let's see if i've learnt anything. w/v=a 500w/12=41.6a plus about 5% say 45amps (although unspecified inefficiency in the microwave means the wattage will be higher. try again. 750/12=62.5 plus 5% 66amps + whatever inefficiency the microwave requires. Q. would an inverter microwave be more efficient?
Now about batteries. To produce the power required by a microwave, the amps input into the inverter is likely to be about 84A. Your 85Ah stop start battery could produce that sort of amps, but only for the sort of time that a starter motor runs for - a few seconds maybe. Taking that amount of amps for several minutes would definitely reduce the life of the battery drastically.
OK and I'm guessing there are batteries designed to allow high output and faster discharge? it's a spare battery, leftover from work done on the car, so for the purposes of inverter science I'm willing to sacrifice it!
To provide long discharges of 85A or so, you would need lead-acid batteries totalling about 5 x 85 = 425Ah. However a microwave typically has short discharges of a few minutes occasionally, you could probably get away with half of that, say two 110Ah batteries, and that's what many people do. The upshot is, you'll need about three 85Ah batteries to power an inverter to run an 800W microwave. Or a couple of 110Ah batteries.
Ok that kind of makes sense.
One of the reasons lithium batteries are so popular is they can be discharged at a higher rate than the same Ah capacity lead-acid battery. A 100Ah lead-acid battery should be restricted to about 20A discharge, but many 100Ah lithium batteries can give 100A discharge without reducing the battery longevity.
Ah ok. So lead acid batteries should have a limited discharge (and can only go down so far?) whereas you can hammer the Li ones and they just keep coming back for more punishment. Like Rocky against Drago in Rocky IV



IMG_20240618_091134.jpg

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Jan 2, 2024
711
940
Lincolnshire, UK
Funster No
100,498
MH
Peugeot boxer
Exp
2020
What batteries do you have in your van and what charging solar?B2B?Split. Charge?Mains?
As for killing your old battery it will but not instantly the more you hammer it the shorter it's life same with discharge don't go more than 50%
 
OP
OP
S
Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
What batteries do you have in your van and what charging solar?B2B?Split. Charge?Mains?
As for killing your old battery it will but not instantly the more you hammer it the shorter it's life same with discharge don't go more than 50%
What batteries. one black one with lots of wires, as per the photo.
charging solar. yes. 2 of them. looks exactly like the photo below.
Split? I don't know.
Charge? Presumably the battery keeps working and the 12v system keeps going.
Mains. not plugged in, mostly camp off grid.



IMG_20240618_095042_edit_776299353261230.jpg
 
OP
OP
S
Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
Mastervolt kit is very nice kit (certainly up there with Victron for quality) - which inverters and charger do you have? You may find that to change the settings you will need an interface to use a USB cable (printer type) and a laptop and app. As oldiesontour has said your photographed batteries will not support a powerful inverter - but at least Mastervolt stuff is pure sine wave output.
I picked up a pair of these plus a mastervolt charger.
What sort of settings would I need to change?



Screenshot_20240618_100016.jpg
 
Apr 27, 2016
7,103
8,367
Manchester
Funster No
42,762
MH
A class Hymer
Exp
Since the 80s
Aaaaah you lost me at Watts. if a 12v battery carries 12w what is the difference between a v and a w?
Each amp carries 12 watts. So if something uses 60 watts, it requires 5 amps from the battery.
If my battery is 85ah isnt that the ability to give 85 amps for an hour. So theoretically it can give the 80 amps needed?
If you look at the small print you will see that the 85Ah capacity is at the '20 hour rate', or C20. So yes, it will give 4.25A for 20 hours. Theoretically it will give 85A for one hour, as you say. But the theory doesn't go that far in practice. If the amps is higher than about a fifth of the Ah capacity then it degrades the battery longevity. That would be about 17A.

If you wanted to get 80A from lead-acid batteries continuously, say for a busy burger van, then you would need about 5 x 80 = 400Ah of batteries. Off-grid houses that rely on batteries will have at least that size of battery bank, and probably more. For occasional microwave power, five minutes here and there, then people find they can get away with less, maybe 200Ah of battery, which is not unusual in a motorhome. But just 85Ah will struggle to power an inverter running a microwave.

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Apr 9, 2022
469
470
Funster No
87,949
MH
Cathargo
Exp
Newbie
Those should be fine as they are as long as the battery capacity feeding them/it is up to snuff - the charger may be the device that needs some settings changed sorry wasn't clear in first post.
 

TheBig1

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Nov 27, 2011
17,794
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Dorset
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19,048
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A class
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many many years! since I was a kid
for battery technology, car starter batteries and leisure are very different. Car batteries are designed to provide high power for a few seconds starting the engine then recharge rapidly from the alternator. They have thin lead plates suspended in acid to hold the charge. Leisure batteries are also known as deep cycle and are designed to provide lower power over an extended period. Leisure batteries have thicker lead plates to handle this current draw cycle between recharging.

If you discharge either too far, it reduces the capacity of the battery each time. Once a battery has been discharged to 10v it is generally done for. Some times they can be recharged using a special charger. What happens in principle is as you discharge a battery, the acid crystalises on the lead plates (called sulphation) and the thicker the layer of crystals, the less it can be recharged

When buying batteries, an important factor is the theoretical recharge cycles. As in how many times it can be recharged from flat. By keeping within the 50% discharge range, you are extending the life expectancy of the battery by only recharging a little at a time. With solar panels, they put a very slow charge into the batteries when the sun shines so extending the number of possible charge cycles. Using a heavy load like an inverter with microwave, you drain the battery fast and repeated use will shorten the life span of the battery by deeply discharging

Yes to those that know the exact details I have over simplified the explanation deliberately
 
Jan 2, 2024
711
940
Lincolnshire, UK
Funster No
100,498
MH
Peugeot boxer
Exp
2020
Running your inverter for short time on your vans batteries would be fine put it as close to the batteries as possible feed it through a fuse saying 150amp and a suitable switch so you can turn off the inverter when not in use.
Looks like apx 200 watts of solar although in practice you won't see much above 75% of that at the best weather and much less on a cloudy day,so long term work towards lithium £200 upwards,maybe a b2b with mppt £200ish and more panels if you want more time off grid 50pence per watt
 
Apr 27, 2016
7,103
8,367
Manchester
Funster No
42,762
MH
A class Hymer
Exp
Since the 80s
OK and I'm guessing there are batteries designed to allow high output and faster discharge? it's a spare battery, leftover from work done on the car, so for the purposes of inverter science I'm willing to sacrifice it!
Starter batteries are designed to give a massive current for a short time, then get charged up again very quickly by the alternator when the engine is running. It has multiple lead plates that are thin, and perforated, to maximise the surface area of the plates. If you run them down until they are flat, the material that accumulates on the plates can fall off and be lost, so the battery loses capacity. Typically a starter battery should not be discharged to less than about 50%.

Proper leisure batteries have thicker plates, and can have extra bits to stop the material shedding from the plates. Gel and AGM batteries have non-liquid electrolyte, where the battery acid is absorbed in either a silicone gel or glass fibre matting, which is also good to stop material shedding. However that means that the maximum current is very limited compared to a starter battery. Typically a leisure battery can be discharged to the 20% level without degradation.
 
Jan 2, 2024
711
940
Lincolnshire, UK
Funster No
100,498
MH
Peugeot boxer
Exp
2020
Beware many cheapo batteries sold as leisure batteries are simply relabeled starter batteries,good quality ones now come in amp for amp the same sort of price as lithium which also have a much greater life expectancy.

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OP
OP
S
Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
Running your inverter for short time on your vans batteries would be fine put it as close to the batteries as possible feed it through a fuse saying 150amp and a suitable switch so you can turn off the inverter when not in use.
Looks like apx 200 watts of solar although in practice you won't see much above 75% of that at the best weather and much less on a cloudy day,so long term work towards lithium £200 upwards,maybe a b2b with mppt £200ish and more panels if you want more time off grid 50pence per watt
Thanks for the advice. Whats a B2B?
 
OP
OP
S
Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
Each amp carries 12 watts. So if something uses 60 watts, it requires 5 amps from the battery.

If you look at the small print you will see that the 85Ah capacity is at the '20 hour rate', or C20. So yes, it will give 4.25A for 20 hours. Theoretically it will give 85A for one hour, as you say. But the theory doesn't go that far in practice. If the amps is higher than about a fifth of the Ah capacity then it degrades the battery longevity. That would be about 17A.

If you wanted to get 80A from lead-acid batteries continuously, say for a busy burger van, then you would need about 5 x 80 = 400Ah of batteries. Off-grid houses that rely on batteries will have at least that size of battery bank, and probably more. For occasional microwave power, five minutes here and there, then people find they can get away with less, maybe 200Ah of battery, which is not unusual in a motorhome. But just 85Ah will struggle to power an inverter running a microwave.
OK that makes sense, thanks for explaining. Would it be opening up a can of worms to suggest connecting the spare battery i have to the existing battery in the moho? in order to be able to run the inverter off both or would that still require a fairly comprehensive restructuring of the whole system?
 
OP
OP
S
Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
for battery technology, car starter batteries and leisure are very different. Car batteries are designed to provide high power for a few seconds starting the engine then recharge rapidly from the alternator. They have thin lead plates suspended in acid to hold the charge. Leisure batteries are also known as deep cycle and are designed to provide lower power over an extended period. Leisure batteries have thicker lead plates to handle this current draw cycle between recharging.

If you discharge either too far, it reduces the capacity of the battery each time. Once a battery has been discharged to 10v it is generally done for. Some times they can be recharged using a special charger. What happens in principle is as you discharge a battery, the acid crystalises on the lead plates (called sulphation) and the thicker the layer of crystals, the less it can be recharged

When buying batteries, an important factor is the theoretical recharge cycles. As in how many times it can be recharged from flat. By keeping within the 50% discharge range, you are extending the life expectancy of the battery by only recharging a little at a time. With solar panels, they put a very slow charge into the batteries when the sun shines so extending the number of possible charge cycles. Using a heavy load like an inverter with microwave, you drain the battery fast and repeated use will shorten the life span of the battery by deeply discharging

Yes to those that know the exact details I have over simplified the explanation deliberately
loving the over simplification!
 
Jan 2, 2024
711
940
Lincolnshire, UK
Funster No
100,498
MH
Peugeot boxer
Exp
2020
Battery to battery charger(charges leisure batteries from engine,can be bought with built in solar controller and starter battery charging from solar when van not in use, don't make the same mistake as many me included and put what may turn out to be too low ampage in I have just changed a 1 yr old 30a for a new 50amp
 

TheBig1

LIFE MEMBER
Nov 27, 2011
17,794
44,217
Dorset
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19,048
MH
A class
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many many years! since I was a kid
Over over simplified, think of a battery like a tank of water. The speed you empty it is the current. So turn the tap on full and it empties quick, slow and it will last longer. An Inverter is like a bigger pipe, so will draw current as fast as it can get it and the tank struggles to keep up with the flow. More tanks (batteries) makes keeping up with the flow easier as they share the load

One thing not mentioned is that many microwaves require an initial surge current to start twice the rated wattage. So it needs more current for a few seconds (more water from the tank briefly)

Think of car batteries as an electric shower, good for a few minutes and leisure batteries like a bath, more water over a longer time. The shower recharges quickly for reuse, the bath takes longer for the water to heat back up

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OP
OP
S
Mar 26, 2021
108
99
Worcestershire, UK
Funster No
80,001
MH
Burstner A645 Active
Here are some pics of the kit i got. I thought even though it's older, its a good brand and for £150 it was hard to justify not getting it! there are also 2 heavy duty batteries from mastervolt too but they were both left for a number of years and one I can't get to more than about 4-5v using the recovery mode on my charger. The other gets up to 12v but under load this drops to 10.... so i'm guessing they are both scrap... at least they weigh about 15kg each so should be worth something for scrap. (every cloud : )



IMG_20240618_113850.jpg

IMG_20240618_114006.jpg

IMG_20240618_114102.jpg
 

TheBig1

LIFE MEMBER
Nov 27, 2011
17,794
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many many years! since I was a kid
Decent inverters and the built in mega fuse is a good idea. Mastervolt kit is pretty good quality. The batteries are scrap at about £15 each. Just taken a pile to the scrap yard the other week
 
Apr 27, 2016
7,103
8,367
Manchester
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42,762
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Since the 80s
Whats a B2B?
There are two ways to charge a leisure battery from the alternator while the engine is running. The simpler way is to use a relay (an electrically operated switch) to directly connect the leisure battery to the starter battery when the engine is running, and disconnect it when the engine stops. That way the alternator charges both starter and leisure batteries, but they are separated when you are parked up. The relay is called a split charge relay.

The second method is to use a proper battery charger that takes whatever the alternator provides (12 to 15V, whatever), and processes it into exactly the voltage and amps the leisure battery needs. That is a bit better for ordinary leisure batteries and a standard alternator, and is essential for a smart alternator and/or lithium batteries. A charger like that is called a Battery-to-Battery (B2B) charger, or maybe a DC-DC charger or DC-DC Booster.

The alternator sends out a signal when it is running and producing charge. This signal (called the D+ signal) is used to switch the split charge relay on and off, or switch the B2B on and off, when the engine starts and stops.

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