Earthing

[EdRedBird]

autorouter
So I have a question on earth and "distributing load"
IF you would have 3 leisure batteries I see the rcommendation to draw power via + on the first battery and - on the last battery ... but what is the point
There is a common earth not ...does it really make a difference

You can reduce the ammount of overal cable if you earth all close tothe decice rather than go all the way back to fuseboxes or powersources not

And iff you do ..why would it make diference if you conect to + on L1 and ground L3 ..or L1 instead

 

[Bucket]

My understanding is that when batteries are linked up to make a bank they are acting as a single source with larger voltage, (same as when you put a number of batteries in say a torch or radio), you must then treat the bank as one, so connect to the beginning and the end, + and -.
If you tried connecting elsewhere inside the bank you could short it with highly dodgy results.

 

[Guigsy]

If you wire with +'ve on one battery and -'ve on the other, if you have a loose connection on any of battery-to-battery connections, you'll completely lose power, but it'll fail safe.

If you draw from the terminals of one battery and just parallel connect to the other batteries, if you get a poor connection, you'll end up with one battery doing all the work.

 

[Richard n Ann]

Also it's to even out the voltage drop across all the batteries. Imagine a long chain of parallel batteries the end one will do less work because of the cable resistances.

 

[NicM]

If you are using a B2B hab battery to engine battery, then they would not have a common grounds anyway.

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[Guigsy]

Also it's to even out the voltage drop across all the batteries. Imagine a long chain of parallel batteries the end one will do less work because of the cable resistances.

With a thick and short wire between batteries, it'll make virtually difference. There's probably more natural variation between cells than you'd get across such a short length of cable. And it'll even out when any load drops off anyway.

I think it's a 'in case of loose connection' safety thing.

 

[Two on Tour]

If you are using a B2B hab battery to engine battery, then they would not have a common grounds anyway.

Why not, my B2B is a non-isolated earth.
Our hab battery grounds to the vehicle chassis as does our starter battery.

 

[Richard n Ann]

The knowledge bot's here .....

 

[Richard n Ann]

The knowledge bot's here .....

Must be busy, he's gone again

 

[andy63]

Interconnecting batteries is well covered here..and you will see that it becomes a real factor as the load current is increased..if its done properly the batteries are more likely to stay better balanced and last longer..
On amodern vehicle with all the electronics and data transfare going on you have to be careful where you use a chassis return point.. if its not done properly it can cause problems..taking a return back to a fuse box can aid and simplify fault finding... at the expense of using a fair bit more wire..
Andy

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

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[andy63]

If you are using a B2B hab battery to engine battery, then they would not have a common grounds anyway.

Don't follow that comment...
Stirling even go as far as recommending that the negatives of start and hab batteries are connected..rather than rely on a common chassis link..
Andy

 

[autorouter]

The idea is to equalise the voltages on all the batteries. Wires have a definite non-zero voltage drop, depending on length and thickness. These voltage differences may be small, but they are constant, and apply throughout the whole life of the battery. For two batteries, if the load/charger is wired to the positive of one and the negative of the other, the voltage drops are equalised.

For more than two batteries that's not true, but it's better than wiring to just one battery and linking in the other batteries. There are various schemes for connecting 2, 4 or 8 batteries in a similar way. But for any other number, including 3, the best way is to wire to a common point, then wire equal links to each battery.

For example a main fuse is often located near the battery bank. Use this as a common point for the positives, with equal wires to each battery. For the negatives, a common connection or busbar can be used, or a common connection to a measurement shunt is another option.

As Guigsy says, the shorter and thicker the wires, the less difference it makes.

 

[Richard n Ann]

if you have a loose connection on any of battery-to-battery connections, you'll completely lose power

That's only true for series connection. Any broken link between batteries in parallel mode will just isolate some batteries or one battery depending where the break is..

 

[stevewagner]

My understanding is that when batteries are linked up to make a bank they are acting as a single source with larger voltage, (same as when you put a number of batteries in say a torch or radio), you must then treat the bank as one, so connect to the beginning and the end, + and -.
If you tried connecting elsewhere inside the bank you could short it with highly dodgy results.

When creating a battery bank in a motorhome you wire in parallel which increases amperage but the voltage remains the same. With a torch that has multiple batteries they are in series which increases the voltage

 

[Guigsy]

That's only true for series connection. Any broken link between batteries in parallel mode will just isolate some batteries or one battery depending where the break is..

View attachment 556598

Yes. It looks like my logic wouldn't work. Any break wouldn't kill the whole chain...

I'm not convinced about the voltage drop argument though. It's going to be hundredths of a volt and only at the highest loads. When the loads drop, the resistance will become negligible and it'll equalise anyway.

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[NicM]

Why not, my B2B is a non-isolated earth.
Our hab battery grounds to the vehicle chassis as does our starter battery.

That's what I said,

 

[Two on Tour]

That's what I said,

If you are using a B2B hab battery to engine battery, then they would not have a common grounds anyway.

Did you ?

 

[Richard n Ann]

Yes. It looks like my logic wouldn't work. Any break wouldn't kill the whole chain...

I'm not convinced about the voltage drop argument though. It's going to be hundredths of a volt and only at the highest loads. When the loads drop, the resistance will become negligible and it'll equalise anyway.

Look andy's link in post 10. It's surprising when the amperage goes up, and the more batteries you have, the more likely you are to have big loads.

 

[Guigsy]

Look andy's link in post 10. It's surprising when the amperage goes up, and the more batteries you have, the more likely you are to have big loads.

Could it be that if you connect the system up to the terminals on battery 1, when you have three (or more) batteries, that the voltage drop along the chain becomes cumulative? So the one on the other end sees the voltage drop from all the cables? I guess that starts to make more sense.

 

[Richard n Ann]

Could it be that if you connect the system up to the terminals on battery 1, when you have three (or more) batteries, that the voltage drop along the chain becomes cumulative? So the one on the other end sees the voltage drop from all the cables? I guess that starts to make more sense.

Yes

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[NicM]

Did you ?

If you look at the drawing, the OP was meaning to have the grounds in the same place on the chassis, I.e., common ground or just local ones at the Battery's to save running the cables to one point.

 

[EdRedBird]

Starnge that

If you are using a B2B hab battery to engine battery, then they would not have a common grounds anyway.

The common ground from my starter battery is connected to the chasis already anything that i connect to the common terminal on my B2B is therefor wired to the chasis.......


Why bother with meters of black?

So why not wire all negatives battery lights and all to the chasis


As for the drop ... 3" of #4 cable between 2 batteries will not cause that much drop

IF you have an Isolated - ..i can understand the point but, my starter battery is hard wired to the chasis already (1st drawing)

 

[autorouter]

If you are using a B2B hab battery to engine battery, then they would not have a common grounds anyway.

You are using the term 'common ground' in a different way to what most motorhomers use it. OK, technically a common ground can be all the negatives drawn together at a single point, and that single point connected to the chassis/earth. Also called a ground star. There are reasons to do this in some electrical installations, especially where radio frequencies are involved and there could be interference and pickup. Broadcast studios or delicate measuring instruments, for example.

However a common ground in a vehicle like a motorhome refers to all the negatives being connected to a substantial metal chassis, at different points, so that the voltage drop along chassis members is negligible. That's what everyone else was thinking it meant, hence the apparent misunderstanding.

 

[EdRedBird]

autorouter Yep 100% with you, understanding wise
So in N00b terms keeping the black cable short and to the metal chasis not only saves money on cabling but also on voltage drops

BUT

It does not stroke with using positive and negative on the first and last battery in your set) as you can go to chasis close to the set

<------ (L1+) (L1-)
(L2+) (L2-)
(L3+) (L3-)------->


OR

Should you keep your leisure battery pack ...."isolated" up to the common star/ negative on the B2B? so you have the same/simmilar drop /draw

Maybe I am overestimating the importance of wiring like the above schematic
But as a N00B seeing that half explained in youtube videos and batteries being an expensive consumable .... It is (a bit) confusing







So nothing wrong with the second picture......
As the starter battery(s) are already connected to the chassis i am connecting my negatives to the chasis eventually anyway the only difference is the length of cable used.

And my word ground/earth here ...I am on rubber wheels which do not conduct do technically ..... common negative would be better

 

[Raul]

Yes you’re right on the ground /earth terminology here. Many do mistake grounding with earthing.
In DC circuits you do not have earth, PE protective earth, but grounding.
Grounding means a common point where all negatives are bonded together.
Earthing means, a third conductor back to supply or to a earth rod, or both as well. This is found in AC circuits, like the supply to your house, or a sub panel supply to a garage or outbuilding. We don’t have earth on vans. The only earth exists on vans is when you have a inverter, and earth conductor is bonded to one of the 230v conductor in the inverter( back to source). Once you bonded to it, becomes a negative, and you have a reference Live and Neutral, the neutral being the one bonded to PE . This is used to facilitate proper function of RCD that monitors the imbalance or current and has a less resistance return path than the human body, protecting against electrocution. Another advantage is that, some electrical equipment will not function without a neutral. So this PE is necessary if you have this type of equipment. Most things happily work without a earth, being double insulated, and don’t need a true neutral.

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[autorouter]

As the starter battery(s) are already connected to the chassis i am connecting my negatives to the chasis eventually anyway the only difference is the length of cable used.

The idea is to equalise any voltage drops between the batteries. So for example you could have a single chassis point near the battery negatives, and connect all the negatives to it using equal length wires. Then connect all the positives to a main fuse with equal length wires. That's a method that works for any number of batteries.

If you have 2 batteries, then an alternative is to connect the chassis to the negative of one, and the positive load to the positive of the other battery, then wire links to connect the two batteries.This will also equalise the voltages. Doing this for 3 batteries won't quite equalise the voltages, but it's better than just connecting chassis and load wire to one battery, and linking in the other two.

In fixed installations they get round this by using a solid copper bar with a very low resistance, and connect all the batteries to it using short thick links. Or even bolt the copper bar directly to the terminals.