unequal capacity house batteries

[chrisbitz]

I have a 105AH battery, and a 90AH battery, linked together in parallel(?).. +to+ and -to-...

Does it cause trouble with charging and discharging, or do they pretty much equal each other out?

 

[TQA]

I have a 105AH battery, and a 90AH battery, linked together in parallel(?).. +to+ and -to-...

Does it cause trouble with charging and discharging, or do they pretty much equal each other out?

This is fine providing they are the same type of plate construction and electrolyte. Trouble will arise if you mix types say AGM with flooded wet cell.

 

[oldbilbo]

This is fine providing they are the same type of plate construction and electrolyte. Trouble will arise if you mix types say AGM with flooded wet cell.

Howzabout mixing 'capacities'? i.e. Both AGM, but 110ah and 66ah....?

 

[pvb]

Despite what purists may insist, there's no practical problem in parallelling batteries of different capacities, as long as they're the same type (ie both AGM, both maintenance free, both wet cell, etc).

 

[pvb]

Personally I wouldn't do it. Different capacity cells in parallel will discharge and recharge at different rates. I borrowed this from another web site about batteries which explains it fairly well:

It is important to use the same battery type with equal capacity throughout and never mix different makes and sizes. A weaker cell causes an imbalance. This is especially critical in a serial configuration and a battery is only as strong as the weakest link.

Imagine a chain with strong and weak links. This chain can pull a small weight but when the tension rises, the weakest link will break. The same happens when connecting cells with different capacities in a battery. The weak cells may not quit immediately but get exhausted more quickly than the strong ones when in continued use. On charge, the low cells fill up before the strong ones and get hot; on discharge the weak are empty before the strong ones and they are getting stressed.

A high-resistance cell, or one that is open, is less critical in a parallel circuit than in serial configuration, however, a weak cell reduces the total load capability. It’s like an engine that fires on only three cylinders instead of all four. An electrical short, on the other hand, could be devastating because the faulty cell would drain energy from the other cells, causing a fire hazard. Most so-called shorts are of mild nature and manifest themselves in elevated self-discharge.

A weak cell will not affect the voltage but will provide a low runtime due to reduced current handling. A shorted cell could cause excessive heat and become a fire hazard.

If you read your quote again, you'll see it refers to serial connected batteries. Connecting different capacity batteries in parallel isn't a problem - the charge is shared according to the different batteries' needs.

 

[Plevier]

Personally I wouldn't do it. Different capacity cells in parallel will discharge and recharge at different rates. I borrowed this from another web site about batteries which explains it fairly well:

It is important to use the same battery type with equal capacity throughout and never mix different makes and sizes. A weaker cell causes an imbalance. This is especially critical in a serial configuration and a battery is only as strong as the weakest link.

Imagine a chain with strong and weak links. This chain can pull a small weight but when the tension rises, the weakest link will break. The same happens when connecting cells with different capacities in a battery. The weak cells may not quit immediately but get exhausted more quickly than the strong ones when in continued use. On charge, the low cells fill up before the strong ones and get hot; on discharge the weak are empty before the strong ones and they are getting stressed.

A high-resistance cell, or one that is open, is less critical in a parallel circuit than in serial configuration, however, a weak cell reduces the total load capability. It’s like an engine that fires on only three cylinders instead of all four. An electrical short, on the other hand, could be devastating because the faulty cell would drain energy from the other cells, causing a fire hazard. Most so-called shorts are of mild nature and manifest themselves in elevated self-discharge.

A weak cell will not affect the voltage but will provide a low runtime due to reduced current handling. A shorted cell could cause excessive heat and become a fire hazard.

That extract is talking about SERIAL configuration which is an absolute no except in emergency.
In a PARALLEL configuration as per the OP it's much less critical although not ideal.
OP had 105Ah and 90Ah, unlikely to be a problem. Battery capacities can easily vary 10% anyway.
Not so keen on oldbilbo's 110 and 66 in parallel, particularly AGM, that's a big gap. Best if they are from same manufacturer and ideally using the same plates, which you might be able to work out from the box sizes.

 

[RobbieW]

The quote refers to both serial and parallel connection (read the last two sections) and gives a valid reason why parallel connecting different capacity cells is not a good idea. Yes less critical than serial connection but still not advisable as when one cell fails, which it eventually will there is a risk of fire. The important point is: 'An electrical short, on the other hand, could be devastating because the faulty cell would drain energy from the other cells, causing a fire hazard.'

Cant see that theres a causal link between the first two paras and the last two. The first two give a good reason to be wary of having unequal capacities in one bank. The last two seem to me to describe a risk generic to batteries, whether there is 1 battery or 'n' connected together. I'm sure theres a higher risk with 'n' as theres 'n' times the current available, to remove that risk would be not to have batteries.

 

[pvb]

The quote refers to both serial and parallel connection (read the last two sections) and gives a valid reason why parallel connecting different capacity cells is not a good idea. Yes less critical than serial connection but still not advisable as when one cell fails, which it eventually will there is a risk of fire.

So are you suggesting that boats shouldn't have more than one battery in a bank? I imagine not. You are seriously overstating the risk of fire.

 

[sailinglegend420]

... If it's OK to mix batteries then why do the battery and boat manufacturers suggest not putting a new battery into a bank of old batteries? That is the same as putting a small capacity battery with an big capacity battery as the older ones will have reduced capacity....

It's not the same at all. Old batteries will have a different terminal voltage at different States of Charge. New battery's terminal voltage will track at all SoCs during both charging and discharging. Parallel old and new and the newer battery terminal voltage will be dragged down by the old battery.

Recent research by Lifeline Batteries has proved that a bank can be made up of different capacities and that the life of the batteries was not compromised, as long as they are all the same make and the same age. This goes against most previous guidelines, but Sabre yachts, which use Lifeline AGMs, regularly ship with different sized batteries to be able to get the maximum capacity from all the available space.

 

[david_bagshaw]

SERIAL configuration which is an absolute no except in emergency.
Are you certain about that, as far as I know know there are very few 24 volt batteries. the common method is to series connect two 12 v ones

In a PARALLEL configuration as per the OP it's much less critical although not ideal.
to get a decent capacity without building the cells in position is to parallel batteries.

Nearly essential they are from same manufacturer and ideally using the same plates and of the same age

So we have 4 12 volt, in two groups, 2x 2 in series to get 24v then joined in parallel to get the greater capacity, Each battery being pretty much at the limit of my ability to struggle it into position in the engine room.

 

[pvb]

My concern would be that by mixing capacities you are increasing your risk of imbalanced cells and hence charging energy would be wasted to heat by the transfer of charge from a good cell to a not quite so good cell during charging. Identical capacity cells surely have a greater chance of having the same internal resistance?

If the batteries are parallelled, the voltage across each cell is identical, regardless of cell capacity. If one cell becomes faulty, the safety risks are essentially the same as a faulty standalone battery - ie minimal - although a faulty cell in one battery in a bank will gradually lead to discharge of the other batteries.

 

[Plevier]

So we have 4 12 volt, in two groups, 2x 2 in series to get 24v then joined in parallel to get the greater capacity, Each battery being pretty much at the limit of my ability to struggle it into position in the engine room.

Nothing wrong with that, my comment you replied to was about batteries of different sizes in series being a no.

 

[Plevier]

As also posted to PVB the quote is referring to serial and parallel - read the last two sections.

I admit to not having read it thoroughly enough!

However the risk of a shorted cell in one bank going up in flames as proposed is absolutely minimal, I'm tempted to say nil. The net voltage across the failed cell is nominally 2V, not battery voltage. Cells rarely short with very low resistance and high current capacity so the power dissipation is unlikely to be great. Furthermore if there is any significant current flowing, the voltage of the good bank which is driving the current will drop, and the voltage of the good cells in the bad bank which are having current driven through them will rise, so the voltage drop across the shorted cell will reduce!
Think about a nominal 12V case. The good bank will supply about 12.8V (open cct voltage fully charged). Consider the bad cell to be shorted. You are then applying 12.8V to a battery of 5 cells. That's 2.56V per cell, a not outrageous boost charge voltage. If any significant current flows the 12.8V will drop.
I have dealt with 400V batteries with 4 parallel banks of 200 cells each and they were not considered a safety risk. It's good practice to fuse each bank separately - not specifically for the shorted cell situation - but a lot of people don't.
The bigger danger of a bad cell is actually when it's on charge. This happened to me this year. After many hours motoring I happened to touch one of the batteries and found it was extremely hot. I disconnected it and later examination showed one cell had a short. The alternator was turfing out 14.4V i.e. something up to 2.88VPC on the remaining 5 cells and pushing substantial current through the battery, developing heat in all the cells. This danger is there whether you have parallel banks or not.

 

[oldbilbo]

My query is now diverging from the OP's, so I'll start a separate thread.....

 

[pvb]

So if a cell in the small capacity battery had a catastrophic short there would be no increase of fire risk when the energy from the larger capacity cells of the other battery or batteries rush through the smaller capacity plates?

No, there's no fire risk.

My question to sailinglegend420 was as to whether there is a increased chance of battery cell imbalance due to the different manufacturing involved to produce smaller capacity cell batteries compared to larger capacity cell batteries even by the same manufacturer. Can you be certain that the internal resistance is identical? Surely if you buy identical capacity batteries from the same production batch you eliminate this risk of imbalance as much as possible.

Not sure why you keep mentioning cell resistance. When batteries of different capacities are connected in parallel, they have the same charging voltage applied to them, and each battery accepts charge according to its characteristics.

We still don't know if the OP is certain that his batteries are even the same age and same manufacturer. I suspect he read the first reply and rushed off happy!

Let's hope so.

 

[pvb]

There is a fire risk as there is with any battery system. Just trying to decide whether having different capacity batteries increases that risk or not. Did you see the magazine article about the Westerly Corsair that was written off due to a battery fire? I bet the owner of that one would love to be told there is no fire risk!

OK, let's agree that there's no significant fire risk involved in having 12v batteries on a boat. If you believe otherwise, maybe you'd be better taking up a safer pastime. Stamp collecting is fairly free of hazards.

 

[billcole]

"One thinks he shorted his accidentally with a spanner and the other one just exploded while being charged by solar panels while the owner was away."

Spanners and batteries have very different electrical characteristics, so what happens when you connect a spanner in parallel with a battery is no indication of what will happen if you connect a battery in parallel with a battery! I wouldn't be surprised if the spanner exploded as well as the battery!

Seriously though, two batteries connected in parallel are a bit like two water butts connected at the base...the water level (analagous to the voltage) will always stay the same in both, and it won't be the case that the smaller of the two will become empty before the bigger one does, and when you charge/fill them up the smaller one won't overflow before the bigger is fully charged. It is true though that if one of your water butts is old and starts to leak it will empty the other one.

Regarding the battery which exploded on solar charge that does show the importance of having a decent regulator to prevent overheating and/or gassing, and also of incuding a suitable fuse near to the battery in case the regulator fails to a short circuit, I suspect it was one of those two possible faults which caused that explosion.