is 2 x 85amh agm batteries enough for a fridge?

[Summitseekers]

I recently finished my van conversion and tend to keep things pretty simple.

I have 2 x 85amh agm batteries- new. A split relay, tiny solar panel powering 2 x USB sockets, 3 lights, a diesel heater and have just added a 12v fridge.

My little one and I plan to live in it for most of the time.

I keep reading that if the battery goes below 11 it means they are knackered.

Last night was our first full night with fridge powered - the digital battery meter on my USB socket said 12.4. This morning it reads 11.1 - does this mean the fridge is draining the batteries and I would have less than 24hrs of off grid power??

I have no water pump, no fan, nothing else hard wired to the batteries and on all the time?

any help would be madly appreciated

 

[Jake Royd]

It will be ok for a day or so providing you drive somewhere so the batteries are recharged (assuming your house batteries are charged by the engine alternator)
Roughly.
Check the current your fridge needs , let's say it's 2A.
You have 170Ah total.
Assume 2/3 of this is usable.
So say 115Ah
That gives you 115hrs.

All just guesstimates.

For long term operation off grid you need as much solar power as you can fit so review how you plan to use your vehicle.

You may have to take the wattage of your fridge divide by 12 to get Amps but then consider how much run time you have per 24hrs to give your Amp hours.
Remember this will vary with weather or internal temperature of you can.

 

[Jake Royd]

Checking further 11.1 indicates your batteries are pretty low.
See below

Google Image Result for https://marinehowto.com/wp-content/uploads/2017/04/02-Loaded-Battery-Voltage-vs.-SOC-AGM.jpg

This is a chart of resting voltage Vs state of charge for AGM.

 

[chaser]

They won't last long if you are not moving or got a bigish solar panel, we have two 100 a battery's and they keep ours going ok but we don't use the batteries for much else when the sun goes down, we have a 100 w solar panel and in a nice day that will keep it all running, no problem.

 

[Garfields]

The diesel heater has a fair draw on power if you have been running that as well

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[peterc10]

Batteries go flat if they are not charged. What are you using to charge yours?

 

[Summitseekers]

It will be ok for a day or so providing you drive somewhere so the batteries are recharged (assuming your house batteries are charged by the engine alternator)
Roughly.
Check the current your fridge needs , let's say it's 2A.
You have 170Ah total.
Assume 2/3 of this is usable.
So say 115Ah
That gives you 115hrs.

All just guesstimates.

For long term operation off grid you need as much solar power as you can fit so review how you plan to use your vehicle.

You may have to take the wattage of your fridge divide by 12 to get Amps but then consider how much run time you have per 24hrs to give your Amp hours.
Remember this will vary with weather or internal temperature of you can.

OK so I need to do soke calculations and see if the batteries will suffice or if I need to increase power somewhere- thanks thats super useful.

 

[Summitseekers]

Checking further 11.1 indicates your batteries are pretty low.
See below

Google Image Result for https://marinehowto.com/wp-content/uploads/2017/04/02-Loaded-Battery-Voltage-vs.-SOC-AGM.jpg

This is a chart of resting voltage Vs state of charge for AGM.

yes this is what I thought but that's odd as I parked up with full charge last night and woke up this morning and it was at 11.1.

perhaps my battery reader isn't accurate? it's only a Chinese USB/12v plug port one.

 

[Summitseekers]

The diesel heater has a fair draw on power if you have been running that as well

I wasn't no which was why I was surprised - I may need to increase my off grid power - I don't have 240v option so

 

[Summitseekers]

Batteries go flat if they are not charged. What are you using to charge yours?

I have an 85w solar panel but live in Wales ... not so much sun at the moment.

I drive I drive van daily so often recharge using the split charger.

thats all I have, no 240v as I don't plan to be many places I can use it.

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[Summitseekers]

They won't last long if you are not moving or got a bigish solar panel, we have two 100 a battery's and they keep ours going ok but we don't use the batteries for much else when the sun goes down, we have a 100 w solar panel and in a nice day that will keep it all running, no problem.

I thought I didn't have much that needed power as I don't use a water pump and only charge my phone as a rule but I think the addition of the fridge might have made quite a big difference

 

[donkey]

What fridge do you have?
Our Waeco 80lt draws some 6-7 amps, so is quite a big drain on batteries.
A few things to minimise consumption: Don't keep on opening the door. Adjust thermostat so it is 'just' cold enough. compressor fridges still need good ventilation at the back.

Geoff

 

[PhilandMena]

Get as much Solar up as you can manage. |Improve your understanding of battery capacity and how to maximise the life of your batteries. Loads and loads of information freely available on U-Tube, Google and even some good advice on here......... and some bad advice too.

 

[Kannon Fodda]

What type of fridge is it - make / model? I strongly suspect the power draw is substantial and you would need a much bigger battery bank, and then there will be a problem of recharging ability.

Standard lead-acid batteries will be damaged if they are regularly discharged below 50%. AGM may be a bit more tolerant. but not much. Indeed a lot of MHF members don't recommend AGM as they find they don't last. If the voltage on a lead-acid / AGM battery drops (no load) to 12V it is already below 50% capacity. A fully charged battery rests at about 12.6-12.7. Battery at 10V is totally flat.

So with your batteries you effectively can use 85Ah. If you were to use that how long to get that energy back? On the best sunny day your 80W solar might just do it, if you are in full sun, but your fridge would still be running, so perhaps not. Driving, what is your split charge relay rating? The manufacturer system in my van had a max charge of 12A, I'd have to drive half way across England for 85Ah. I now have something called a B2B rated at 60A (with some very thick connecting cables).

Edit - I see you have used your 5 free posts. You'll need to subscribe as a member £15 to be able to get more assistance from us.

 

[peterc10]

OK so I need to do soke calculations and see if the batteries will suffice or if I need to increase power somewhere- thanks thats super useful.

There is no point getting more batteries until you get something that can charge them. More solar is a good start. But you will have to drive long distances if you are relying upon your split charger unless you get a B2B fitted as well.

BTW you have now reached your limit of 5 free posts. If you want to post again you will need to subscribe. It will be the best £15 you will spend on your motorhome with all the good advice on here.

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[Just smiffy]

I have an 85w solar panel but live in Wales ... not so much sun at the moment.

I drive I drive van daily so often recharge using the split charger.

thats all I have, no 240v as I don't plan to be many places I can use it.

You’ll need to drive for around 3hrs to put any decent charge back in your batteries, you need more solar and a good MPPT controller…. As much panels you can get on the roof and as much as you can afford…

oh, and welcome…

 

[Tombola]

fridge = more solar and/or a good b2b charger

 

[autorouter]

Think of your batteries as a temporary store of energy, with the fridge etc taking energy out and the various chargers putting energy in.

First work out how much energy the fridge takes out. It depends on the make/model. The compressor will draw a certain current, say 2 amps, when it is running. However because of the thermostat it may only be running for 50% of the time. That's why manufacturers usually give an 'average energy consumption over a 24 hour period'.

So let's suppose the average consumption is 360 watt-hours per 24 hours. You can easily translate that into amp-hours. The battery voltage is 12 volts, so 360 Wh is 360/12 = 30 Ah. On this forum, people always talk about amp-hours rather than watt-hours, because motorhome voltage is almost always 12 volts.

So the fridge is taking 30Ah out of the batteries every 24 hours. You'd like to put back at least 30Ah into the batteries by charging. Your choices are mains charger, solar, engine alternator or portable generator.

You can get over 40Ah from a 100W solar panel on a good day, but on average it's best to plan for about 30 to 35Ah per day from a 100W panel. You can see that a single 100W solar panel will just about run that fridge, leaving nothing for anything else. That's why 'as much solar as you can fit', or at least 200W, is suggested.

If your engine alternator is charging at 10A, you'll need to drive for 3 hours to put in 30Ah of charge. You can get a 'Battery-to-Battery' (B2B) charger of 20, 30 or even 60A, which will be much faster. This option is good if you drive around every day or every couple of days.

Bigger batteries will help smooth over a run of bad weather or staying an extra day without driving, but it's the charging methods that are important if you have a constant battery drain like a fridge.

 

[mitzimad]

you need to look at total load did you watch tv for several hours ? or other loading? what lighting are you using we had some fluorescents in one van could kill a battery in a few hours