Connecting batteries

[Eclipse Custom Vans]

Thanks, will put it in as close as I can to the panels. Though will be once the cables are inside the roof!

Yes I've opted for thicker cable than needed really to give me peace of mind, and kept the fuses toward the lower end. There is something like a minimum of 10A + between fuse value and cable capacity.

Thicker cable will also help with Volt drop but these are specific calculations. Vans are quite small so VD is not typically an issue, only if on are scrimping on cable sizes, i.e using the smallest you can get away with on amps/load which means increased VD.

 

[Sniggib]

It was for a military communications vehicle and they needed 60mins back up power for batteries. So not your typical camper

10mm cable will take 70-80 amps so I would up the fuse size to prevent nuisance tripping. I would keep all the same cable sizes with the same fuse for simplicity. Aw rarely go larger than 50A on campervans.

Ah, now it makes more sense haha.

That's what I have aimed for with cable sizing, though perhaps gone a bit overkill as per my last post RE cable thickness. This was also to help with voltage drop to keep low. Though on the 12v planet calculator I used it was never an issue!

Essentially all smaller circuits are 2mm wire.

Then wires from battery to fuse box are 10mm, which also matches the 10mm fridge wire.

and from this thread I will use 25mm wire for all of the battery connections.

Fuses do vary a bit though. Having ordered these I will use them and see how i get on with nuisance tripping. As a novice to auto electrics choosing a fuse size proved to be tricky! O One of those professional / experience based judgement things which clearly I don't have! I just made sure they were slightly above my intended amp draw and well within my cable capacity.

Cheers,

 

[Eclipse Custom Vans]

It was for a military communications vehicle and they needed 60mins back up power for batteries. So not your typical camper

10mm cable will take 70-80 amps so I would up the fuse size to prevent nuisance tripping. I would keep all the same cable sizes with the same fuse for simplicity. Aw rarely go larger than 50A on campervans.

here is part of the installation

 

[Eclipse Custom Vans]

Ah, now it makes more sense haha.

That's what I have aimed for with cable sizing, though perhaps gone a bit overkill as per my last post RE cable thickness. This was also to help with voltage drop to keep low. Though on the 12v planet calculator I used it was never an issue!

Essentially all smaller circuits are 2mm wire.

Then wires from battery to fuse box are 10mm, which also matches the 10mm fridge wire.

and from this thread I will use 25mm wire for all of the battery connections.

Fuses do vary a bit though. Having ordered these I will use them and see how i get on with nuisance tripping. As a novice to auto electrics choosing a fuse size proved to be tricky! O One of those professional / experience based judgement things which clearly I don't have! I just made sure they were slightly above my intended amp draw and well within my cable capacity.

Cheers,

2mm ok for smaller circuits. 10mm is quite large depending on the demand, 6mm would be more than adequate typically. Fridge can be 2,5mm if close to battery but 4mm better or 6mm if far from battery, 10m seems too large to me. 16mm prob ok for battery connections by 25mm is bely and braces approach so ok.

Nuisance tripping I mean if fuse too small it will 'blow' unnecessarily and will typically do this when its dark or raining....

Al the best.

 

[Phil_G]

Where are you getting this potential 90A figure from Sniggs?
the amount of current a battery can supply is limited only by its internal resistance, it has nothing whatever to do with your batteries being 90Ah
If you were to parallel an empty battery with a full one, which you never ever do, there will be a large balancing current flow from the full one to the empty one. This current surge rapidly decays with an exponential curve and ends up at zero, and remains so until one of the batteries dies several years down the line.
The surge current is dependent on a few variables but the amphour capacity of the batteries isnt one of them!
I beg to differ re Eclipse Campers who advises a fuse on a solar panel which is an inherently current-limiting device, yet doesnt fuse between batteries capable of huge currents should a plate buckle or disintegrate. A panel fuse is harmless but unnecessary, but I would definitely use fusible straps to join the two batteries.
Cheers
Phil

 

[Eclipse Custom Vans]

Where are you getting this potential 90A figure from Sniggs?
the amount of current a battery can supply is limited only by its internal resistance, it has nothing whatever to do with your batteries being 90Ah
If you were to parallel an empty battery with a full one, which you never ever do, there will be a large balancing current flow from the full one to the empty one. This current surge rapidly decays with an exponential curve and ends up at zero, and remains so until one of the batteries dies several years down the line.
The surge current is dependent on a few variables but the amphour capacity of the batteries isnt one of them!
I beg to differ re Eclipse Campers who advises a fuse on a solar panel which is an inherently current-limiting device, yet doesnt fuse between batteries capable of huge currents should a plate buckle or disintegrate. A panel fuse is harmless but unnecessary, but I would definitely use fusible straps to join the two batteries.
Cheers
Phil

I did actually say ' There is no harm in fusing in case there is a problem with the batteries but unlikely'. The solar panel by definition is remote from the battery so the fuse will also protect against cable damaged, chafed etc. (not that this should happen of course). Incidentally, every solar panel we have ever fitted has indicated a fuse in the instructions, but hey ho !!

All the best

 

[Sniggib]

Thanks @Phil_G

The 90A was me working on the basis of 90A max difference between the batteries. From Deaky's post I now know that is incorrect (unless theoretically a full and empty battery were connected which might cause full draw momentarily and then a reduction as they balance out (thanks for that info)_

So, I now know that the reality is that the batteries will only ever draw a much smaller load as they balance out. I'm going to go with 25mm2 cable (as peace of mind for me!) and will also fuse it at 125A?, so still a big different to the 250A on the cable.

Hopefully I'm on the right track with this last piece of the jigsaw now! Found getting the right cable for battery connection very confusing.

Cheers,

 

[Phil_G]

...so the fuse will also protect against cable damaged, chafed etc.

Sorry, no it wont If you fuse a panel, the rating needs to be sufficient for the panels absolute maximum current in direct sunshine, at the maximum current point at full load. For this reason the fuse can never blow, there is no more current to be had, even into a short.
There is nothing to protect in a panel, they have a relatively high internal resistance and inherently current-limit even quite happily into a direct short-circuit which is how the Isc spec is measured.

 

[Eclipse Custom Vans]

Ha, I read you were using 1000w x 2 solar panels....(quite large for a van....). For smaller panels yes cables should be large enough to handle current but as you are wiring in parallel this will increase current, here is good info for multiple panels in parallel. (American specific but informative - how-properly-fuse-solar-pv-system - Web). While you may not necessarily need a fuse for per se we fit them anyway just in case.

I’ve also attached a photo of a brand new Transit custom we are working on and the 2 batteries they OEM install have no fusing between battery links.

It can be confusing with differing apinions - which you will get on forums, however if you feel happer instaling fuses, do so, you will never be caught out installing more fuses than needed.

All the best,

Ian

 

[Sniggib]

Thanks @Eclipse Campers and @Phil_G

Can I just check, I will install a fuse between the two batteries (being a novice eletrician I am erring on the side of caution for everything!)

I will use 25mm2 wire between the two. I now know that the balance draw from both batteries will be far lower than 90A. However, I am still unsure as to what the general draw will be between both batteries. They will be wired in parallel to aid balancing. Does the fuse between the batteries need to match my maximum expected system load? or again is it just enough to allow the batteries to balance?

So, in essence what size fuse should go in between the two batteries? and ideally why (so I can continue learning!)

Many thanks,

 

[Deaky]

Thanks @Eclipse Campers and @Phil_G

Can I just check, I will install a fuse between the two batteries (being a novice eletrician I am erring on the side of caution for everything!)

I will use 25mm2 wire between the two. I now know that the balance draw from both batteries will be far lower than 90A. However, I am still unsure as to what the general draw will be between both batteries. They will be wired in parallel to aid balancing. Does the fuse between the batteries need to match my maximum expected system load? or again is it just enough to allow the batteries to balance?

So, in essence what size fuse should go in between the two batteries? and ideally why (so I can continue learning!)

Many thanks,

Your batteries are side my side. The interconnecting cables do not need fusing. Make them as short as possible. It’s good practice to take the +ve from one battery and the -ve from the other. Providing both batteries are in the same state of charge when connected there will he only negligible ‘balancing’ taking place. The current flowing through the interconnecting cables will never be greater than the load.

it goes without saying that the +ve cable from the battery bank to the fuse box should be fused closely to the battery.

 

[Phil_G]

...I now know that the balance draw from both batteries will be far lower than 90A...

You keep mentioning 90A - where are you getting this 90A figure from?

 

[t0mb0]

I realise this probably isn’t a helpful question at this point, but....

Why do you want two batteries anyway? Your schematic looks like a pretty standard set of loads and most people are absolutely fine with one. You could get a single 130ah AGM, for instance?

 

[oldiebut goodie]

You keep mentioning 90A - where are you getting this 90A figure from?

I think that it is from the fact that each battery is 90Ah.

 

[Pete C]

I think that it is from the fact that each battery is 90Ah.

That’s what I understand from the flow of conversation above.
@Sniggib - you can’t compare Ah and Amps.
If you connected an “empty“ 90Ah battery to a full one, the current flow would initially be way in excess of 90 Amps.

You are overthinking this battery connection thing. Once connected together, the “balancing“ flow of current between them is irrelevant. As @Deaky says, worry about the load current, which is way less than 90A.

Pete

 

[Sniggib]

You keep mentioning 90A - where are you getting this 90A figure from?

@Phil_G It is from the 90Ah, getting confused between Amps and Ah again!

Still cant get my head around this.

I get that an Amp is a unit of charge and that an Amp Hour is one amp supplied for one hour.

So my LED lights are 0,72A, if I run that for 1 hour I will use 0.72Ah.

Where I go wrong is on batteries. If it has a rating of 90Ah, this means it can supply 90A for 1 hour or 1 amp for 90 hours (or something in between)? This is why I thought it could in theory dump all 90A in one as a maximum and hence my wiring or fusing to account for that. However, it seems I am wrong in this and that the battery can go above 90A in order to discharge is 90Ah in one (theoretically) so I'm now not really sure what Amps rating you would assign to a 90Ah battery.


I think I may be looking at this the wrong way round. What I am thinking is; If the battery shorts what is the maximum amps it will produce, I then base my wiring and fuses on that to protect the system. So for batteries this is more difficult to work out due to the 90Ah rating.

Perhaps I need to think about it the other way. What is the expected loading I will draw from those batteries on a typical use. Then size my cables and fuses to that. If something then shorts, the fuses will blow. So in a sense it is irrelevant how much the battery can produce in Amps in one go. The only problem with this is for the wire between batteries because the required load was unknown (wrongly thought to be 90A) I now know this balancing load will be relatively low, though I am still not certain what this would be or how to work it out. Unless this load relates to my expected current draw as @Pete C eluded to?

That’s what I understand from the flow of conversation above.
@Sniggib - you can’t compare Ah and Amps.
If you connected an “empty“ 90Ah battery to a full one, the current flow would initially be way in excess of 90 Amps.

You are overthinking this battery connection thing. Once connected together, the “balancing“ flow of current between them is irrelevant. As @Deaky says, worry about the load current, which is way less than 90A.

Pete

@Pete C Thanks. I mainly just want to understand the relationship between the batteries. I don't think what I am putting in is 'wrong' but id like to know why its right

I'd of thought my total load on the batteries is only relevant to the fuse size (and cable size)after the battery bank? How would the load current affect the batteries other than draining them rapidly if it is high?

I realise this probably isn’t a helpful question at this point, but....

Why do you want two batteries anyway? Your schematic looks like a pretty standard set of loads and most people are absolutely fine with one. You could get a single 130ah AGM, for instance?

@t0mb0 No worries, Its so I can get more Ah as all of the camping I do will be non-EHU relying purely on split charge and solar. Working on a basic load of 15Ah per day in winter (with no solar input) then the two batteries will get me approx. 6 days at a 50% DoD without needing to turn the engine on. If I switch the fridge on (though less likely in winter) then that will jump the load up a fair bit I think so will obviously then only get a few days. In summer, the plan is that the solar will give enough to run the fridge.

A single 130Ah battery would only have given me 65Ah so 4 days perhaps.

Two batteries will also hopefully mean no needing to run the engine / need to do longer journeys to charge up which can be done on the way home and also should give me a longer overall battery life.

It was a difficult decision to make, but I decided that if two batteries were needed then I would rather buy two now than buy one, find out its not enough and then need to re-buy two to make sure they are of the same age / usage etc.

Thanks all

 

[Phil_G]

Put this 90A thing to bed!
Ah has no relation to how much current a battery can give, that is determined solely by the battery internal resistance
For example a battery having a high Cold Cranking Amps spec will have a very low internal resistance - its Ah capacity doesnt come into it.

@Phil_Ggetting confused between Amps and Ah again!
I get that an Amp is a unit of charge and that an Amp Hour is one amp supplied for one hour.

No its not
an amp hour is a unit of charge. An amp is a unit of current.

 

[andys]

I’m always puzzled when someone says they won’t use the fridge in the winter. Is that a thing? Do people not do that? I use mine just as much, I still need to keep food/milk etc fresh, still need drinks cold and I sure as hell don’t keep the inside temperature of the van down to the 4°C of the fridge.

 

[Sniggib]

Put this 90A thing to bed!
Ah has no relation to how much current a battery can give, that is determined solely by the battery internal resistance
For example a battery having a high Cold Cranking Amps spec will have a very low internal resistance - its Ah capacity doesnt come into it.
No its not
an amp hour is a unit of charge. An amp is a unit of current.

Agggghhhh!!!! Haha.

Ah is a unit of charge. A is a unit of current.
Ah is a unit of charge. A is a unit of current.
Ah is a unit of charge. A is a unit of current.
Ah is a unit of charge. A is a unit of current.
Ah is a unit of charge. A is a unit of current.

Thanks for the info on batteries.

I’m always puzzled when someone says they won’t use the fridge in the winter. Is that a thing? Do people not do that? I use mine just as much, I still need to keep food/milk etc fresh, still need drinks cold and I sure as hell don’t keep the inside temperature of the van down to the 4°C of the fridge.

I plan on putting food outside, or taking ice packs and leaving them outside and then swapping them into the fridge to keep it cool. I am planning a fair few Scotland trips in winter so I think it will be a case of using the fridge to stop the milk freezing outside! Though the extra battery capacity is for autumn and winter so we can run the fridge intermittently whilst we still only get limited solar, but its too warm outside to keep milk etc.

Last Scotland trip;