Mixing Battery types in 1 Bank

[monte]

Hi, I did a quick search but couldn't find an answer so....
I have a new boat with 4 140Ah batteries and I would like to add another 2/140Ah batteries. I would like to use the same batteries but they may not be available. What is the most important thing to consider when adding batteries? I know the best practice is to buy the whole bank at once from the same batch, but this is not an option, either is throwing the ones I have and buying 6 new..
The batteries are Exide ER660 , 660Wh, 140Ah, 750Ah 12V
If I have to select another brand, what is the most important features to match and what is the downside of not having an exact match from the same batch?
Thanks electric guru's

[skipmac]

Yes the ideal in a perfect world is to always have a brand new set of identical batteries. In the real world that often would be a dumb idea if it meant to toss some perfectly good batteries just to add one or two new ones. If the old batteries are on their last legs anyway, maybe toss them but otherwise as long as the batteries you keep are pretty new, in very good condition and haven't been abused then it makes more sense to keep them.

The main thing in adding batteries to an existing bank is to use the same battery type. So don't mix flooded lead/acid with AGMs with gels, etc. Unless one of the old batteries is failing and drawing a lot more power than it's partners you should be OK.

[Andina Marie]

I'm not sure why you would have to match chemistry. The only thing to watch would be the setting on a charger that allowed choice of battery type, in which case you should select one that is compatible with the most sensitive parameter, typically maximum voltage.

Even if the charge level verses voltage correlation is different, a modern charger in bulk mode will be sufficiently high enough voltage to charge them all fully. During discharge some may bear slightly more of the load than others at certain voltages while the others will pick up the load when the voltage gets to their range.

[skipmac]

Originally Posted by Andina Marie

I'm not sure why you would have to match chemistry. The only thing to watch would be the setting on a charger that allowed choice of battery type, in which case you should select one that is compatible with the most sensitive parameter, typically maximum voltage.

Even if the charge level verses voltage correlation is different, a modern charger in bulk mode will be sufficiently high enough voltage to charge them all fully. During discharge some may bear slightly more of the load than others at certain voltages while the others will pick up the load when the voltage gets to their range.

Let me preface my reply by saying that I know just enough about batteries to be dangerous. That being said, I made my recommendation based on the following thoughts and my regular err on the side of caution philosophy.

- You comment that charger voltage on any type setting is sufficient to fully charge any battery type. What about the situation for a typical off the grid cruiser who cycles the house batteries from 50% to 80%. Could that not leave some batteries in a mixed bank undercharged?

- Would not different battery types show a different resistance to the charge voltage leading to higher charge current in some batteries than others again resulting in different levels of charge and potentially running some batteries hotter than others?

[Cheechako]

I'd just get ones as close to the specs of the others as you can get. AH's etc.
On one boat I mixed Gel 4D's with wet cells to no apparent problem. Not that I'm advising it...

[Andina Marie]

Originally Posted by skipmac

Let me preface my reply by saying that I know just enough about batteries to be dangerous. That being said, I made my recommendation based on the following thoughts and my regular err on the side of caution philosophy.

- You comment that charger voltage on any type setting is sufficient to fully charge any battery type. What about the situation for a typical off the grid cruiser who cycles the house batteries from 50% to 80%. Could that not leave some batteries in a mixed bank undercharged?

- Would not different battery types show a different resistance to the charge voltage leading to higher charge current in some batteries than others again resulting in different levels of charge and potentially running some batteries hotter than others?

Your first scenario has some merit however the differences in charge/voltage profile for lead based batteries are so similar that the differences in load sharing would be insignificant compared to normal battery differences in characteristics. But you are correct that the chemistry should be similar and mixing with Lithium batteries would not be advisable.

On your second comment, that is 100% correct but it is normal for the current of even identical batteries to be different depending on their internal series resistance. They won't run any hotter than they would if being charged individually, the individual batteries don't "know" they are in parallel with strangers and react aggressively, all they see is a charging voltage and absorb charge current in proportion to the applied voltage. That voltage they see is probably lower than it would be had they been the only battery on the charger so in fact they would run cooler.
Again there would be a problem mixing with totally foreign batteries like Lithium etc.

[skipmac]

Originally Posted by Andina Marie

Your first scenario has some merit however the differences in charge/voltage profile for lead based batteries are so similar that the differences in load sharing would be insignificant compared to normal battery differences in characteristics. But you are correct that the chemistry should be similar and mixing with Lithium batteries would not be advisable.

On your second comment, that is 100% correct but it is normal for the current of even identical batteries to be different depending on their internal series resistance. They won't run any hotter than they would if being charged individually, the individual batteries don't "know" they are in parallel with strangers and react aggressively, all they see is a charging voltage and absorb charge current in proportion to the applied voltage. That voltage they see is probably lower than it would be had they been the only battery on the charger so in fact they would run cooler.
Again there would be a problem mixing with totally foreign batteries like Lithium etc.

So you're saying I've got the basic theory right but the significance in the real world would be, well, insignificant.

And I was basing my comments on FLA, AGM and gels and leaving out Lithium, Ni and other more exotic varieties.

Just in general I am sticking with FLA batteries for now. I think they give you the most bang for the buck with the least attention, suffer abuse better than other types and if you screw up and kill a couple replacement is much less painful to the wallet.

At some time when the technology gets to the point that its a lot cheaper, it's plug and play and the system doesn't need frequent careful monitoring I'll go with LiFePO4.

[hellosailor]

Monte-
"I have a new boat with 4 140Ah batteries "
So presumably these are new or really close to new batteries. In which case, I'd just buy two more of the same kind. Exide are pretty widely sold and it should not be hard to find them. "Dual" means the plates are not as thick as true deep cycle, not as thin as SLI, so I'd want to buy two more "Dual" batteries because how the plates age, how the batteries age, is going to be affected by plate thickness as well as the battery chemistry, and again, matching what you have will give you the best chance of seeing them all age the same way.

Even among plain flooded LA batteries, there are differences in chemistries and you may find one brand saying the optimum voltage is 1/10th or 2/10ths different from another brand. In the real world the odds are pretty much anything CAN work...no one has yet predicted exact battery life for anything anyway.

But buying two more of the same, would seem to be the most likely way to keep all of the batteries happy, and aging gracefully together. Exide shouldn't be a problem to find, competitively. You might poke around, they may be supplying Sam's Club or WalMart under their own labels at steep discounts.

[belizesailor]

Originally Posted by Andina Marie

I'm not sure why you would have to match chemistry. The only thing to watch would be the setting on a charger that allowed choice of battery type, in which case you should select one that is compatible with the most sensitive parameter, typically maximum voltage.

Even if the charge level verses voltage correlation is different, a modern charger in bulk mode will be sufficiently high enough voltage to charge them all fully. During discharge some may bear slightly more of the load than others at certain voltages while the others will pick up the load when the voltage gets to their range.

Different battery types (chemistries) have different charge profiles including diff charge acceptance rates, and max charge voltages. So of you chose a compromise profile, one of the battery types is going to suffer (at least in the long term).

I've temporarily mixed flooded and AGM before to work around problems. The charge profiles are generally similar. But, corrected the discrepancy in a short period of time (weeks).

From a pragmatic perspective, why experiment with battery chemistry and charge profiles when buying batteries of the same general chemistry is easy (in "developed" countries at least)?

To paraphrase Bogart, I think if you mix battery chemistries & charge profiles "...you'll regret it. Maybe not today. Maybe not tomorrow, but soon and for the rest of your [battery bank's] life.”

[colemj]

Mixing flooded and AGM's can probably be done with impunity since their charging profiles are so similar and one almost never gets to the higher voltages that flooded will take for very long.

Mixing flooded with gel is a different story. I am surprised Andina didn't check herself on that post. If we mixed gel with flooded on our boat, we would surely ruin the gels within a couple of weeks, as the charge profile for our flooded batteries is an absorption voltage of 14.8V and we reach that often.

In the OP's case, his batteries are mantainence-free flooded batteries. Not the best solution for a house bank on a boat, but that is what he has. Any flooded or AGM battery will work well with those - though an AGM would be a silly addition.

He doesn't tell us the age of the existing batteries. If they are older than 4 years and he plans on cruising long-term, I personally would chuck them all and buy better ones. If this is a stop-gap addition or he is happy to be in a continual rotation cycle with the bank, I would go with any deep cycle battery at a good price. There really is nothing to match regarding capacity, size, etc.

Mark

[westwinds]

If you have flooded cell with caps for adding water, get a glass not plastic, hydrometer and determine if the batteries are completely charged, if not is a complete charge possible.

[monte]

Thanks for the feedback. The batteries are 1 month old but only used for a 6hr sail. Apart from that they have been plugged into shore power and fully charged. Exide sell a model called er650 which is 142Ah 12v but the Wh is 330 and I read that duel batteries are rated by Wh. Also it is a lot smaller. Not sure what's up with that ?

[colemj]

That ER650 will be fine. The Ahr and Whr ratings are all dependent on which discharge number they use. From a quick google, it looks like these two are similar batteries, but be aware that the ER650 is a different physical size and has both terminals at one end of it. The 660 appears to be the newer model.

Mark

[valhalla360]

Unless you have done something bad to them, 1 month old should effectively be new. Match the chemistry and move on.

If they were older (say 4-5yrs), another option would be to wire them up as a second battery bank.

[hellosailor]

Monte-
Exide's web site doesn't know your ER660 batteries exist.

I'd say call them directly:
Headquarters
Building 200
13000 Deerfield Parkway
Milton, Georgia 30004
USA
1.678.566.9000 - phone
1.678.566.9188 - fax

and see if you can obtain two more, or if that's some special "to the trade only" label on another product.

If those are simply "maintenance free flooded wet acid" batteries, you have my condolences. That's a bastardized design that simply doesn't allow routine maintenance, but still really requires it. GM/Delco tried it in the 70's and it didn't work out so well for them, since it still needs maintenanace (less than unsealed) but simply won't allow it unless you chew out the plugs.

I'd still invest the phone call and try to get two more "same same" if they're not ridiculously expensive. Otherwise, I'd agree, local conventional wet acid would be close enough.