Parallel Batteries with different Ah ratings

I have 4 house batteries that are also used as starting batteries on my boat. It was wired this way from the factory.

Recently I replaced my flooded cell batteries with AGM's that have an Ah of 66/hr. Run in a parallel this gives me a theoretical 264Ah.

Is it possible to add a third battery of the same voltage but with a higher Ah (8D maybe) to this parallel to increase my house bank Ah?

I'm leaning towards it not being possible as they would discharge at different rates but wanted to double check my math.

Possible but not recommended.

The ideal is all the same age as well, but if the rest really are recent and in good shape, get two more matching to add 132AH and you should be fine.

k9medic said:

I have 4 house batteries that are also used as starting batteries on my boat. It was wired this way from the factory.

Recently I replaced my flooded cell batteries with AGM's that have an Ah of 66/hr. Run in a parallel this gives me a theoretical 264Ah.

Is it possible to add a third battery of the same voltage but with a higher Ah (8D maybe) to this parallel to increase my house bank Ah?

I'm leaning towards it not being possible as they would discharge at different rates but wanted to double check my math.

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john61ct said:

Possible but not recommended.

The ideal is all the same age as well, but if the rest really are recent and in good shape, get two more matching to add 132AH and you should be fine.

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My understanding on this is so poor.... Isn't the issue with battery age related to the change in internal resistance over time? Wouldn't this also be the reason to stick with batteries from the same manufacturer and size?

What I don't understand is what is the effect of a difference in internal resistance on the total battery bank. What bad stuff does that do?

In the OP's case, if he wires his existing 4 batteries in parallel properly, then treating that as one 264Ah battery, he then parallels that to an 8D with 255 Ah shouldn't that work out reasonably well?

His question of paralleling unequal sized batteries, and new vs old is one that I'd really like to understand. We always here that it is bad practice, but I've yet to see an explanation that is simple enough for my feeble mind to wrap itself around.

dimer2 said:

https://www.homepower.com/articles/...llation/ask-experts-batteries-series-parallel

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That is one of the articles that I've read as well in looking at what I want to do. However the authors last sentence says this;

If the system tends to operate at less than a full state of charge, adding new batteries to old will probably just result in the old ones pulling the new ones down and everything getting sulphated.

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Emphasis mine. I don't get it.

However connected, if some components of the system are made to carry more of the load they get weakened more quickly.

When things aren't balanced, some overcharged/undercharged over/under-loaded, the system as a whole will function as effectively as the weakest link.

In this scenario the age difference should be minimal, but it is best not to mix different model batteries with different construction and performance characteristics.

Thanks John.

Interestingly, if you have a VCR, or ACR, or other such paralleling relay to piggyback charging of your start batteries off your house charger, you are doing EXACTLY what the experts say not to do. You are wiring two dissimilar batteries together and changing them as a single bank. Yet this approach is very common, and the world hasn't come to an end as a result.

So why isn't it recommended? It's not ideal because depending on the battery type, capacity, and age, different batteries will take different amounts of time to reach full charge. When the batteries are all the same, they will theoretically take the same time to reach full charge, so none get over charged or under charged.

Now, throw in a different battery into the bank. It might require much less time at charging voltage to reach full charge, yet the charger keeps running to get the other batteries up to full charge. So, for example, if you are getting underway with a low house bank, it will take a long charge underway to get back to full charge, yet a connected start battery will reach full charge very quickly. So the start battery gets over charged. Fortunately lead acid batteries are pretty tolerant of over charging, so we get away with this.

Then consider when the odd battery requires more time to recharge. The charge will conclude that charging is complete when the full bank appears charged, but the odd battery will not quite be there yet, so it gets left partially charged. This might happen if you have a heavily used thruster battery that gets charged in tandem with a house bank that is already charged when you leave the dock. Charging completes quickly because the house bank if already full, and the thruster battery doesn't get the full charge that it requires.

Yes, ACR and the other voltage following combiners connect "raw", but only during charging cycles.

The charge sources are supposed to sense voltage at and output current direct to the House bank, with charging Starter really less critical.

With solar or all-day motoring yes more of an issue, especially if both banks are expensive and of a chemistry with different requirements.

You should then move up to DC-DC charging, or charge sources with multiple independent outputs.

Some like Echo Chargers limit both current and voltage levels.

Twisted, that is why I have always liked something like the Echo Charger or Duo Charger. I just swapped out the Blue Seas ACR for the Echo Charger for my start battery. I didn't see any sense in combining a house bank of 3 old SLA batteries with 1 newer 8D Lifeline start battery.

The Duo Charge messed with the electronic engine, but the Echo Charger is behaving nicely. The only downside with the Echo Charger is that you can't specify charge voltage to the start battery. Max charge current is 15 amps. The Duo Charge can set charge profiles independent of what is charging the house bank.

dhays said:

Twisted, that is why I have always liked something like the Echo Charger or Duo Charger. I just swapped out the Blue Seas ACR for the Echo Charger for my start battery. I didn't see any sense in combining a house bank of 3 old SLA batteries with 1 newer 8D Lifeline start battery.

The Duo Charge messed with the electronic engine, but the Echo Charger is behaving nicely. The only downside with the Echo Charger is that you can't specify charge voltage to the start battery. Max charge current is 15 amps. The Duo Charge can set charge profiles independent of what is charging the house bank.

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For all intents and purposes,the Echo Charger is the same as an ACR. It's output just follows the output of the input side, with current limiting to 15A. So the charge voltage and duration follows the input side regardless of what the output side needs.

Th DuoCharger is actually a 3 stage charger that happens to be DC powered rather than AC. In that respect it is much better since it will taylor the charge voltage and duration to what the battery needs.

You uncovered a nasty noise issue with the Duo that I hadn't heard about before. It has what I would consider to be another nasty flaw; if the battery being charged wants to draw more than the Duo's rated 15A, rather than current limiting at 15A, the Duo shuts down. It restarts after a while, but will continue to shut down until the battery is charged enough to no longer draw more than15A. That means if the Duo is the only charge source, the battery will never get recharged. So when your battery is the lowest and needs the most charge, is exactly when the Duo can't charge it. How silly is that?

I'm not really a Mastervolt fan, but they have a DC to DC charger that works like the Duo, but current limits rather than shutting down. It's called the MAC something or other. Thats what I ended up using in this situation. The down side is that it's a bit pricey - around $400 if I recall. I think all the other approaches cost $200 or less.

The Echo Charger's max of 14.4V should be just right for a normal Starter.

Of course if your House bank wants lower than that from shore or alt charging, then that's what Starter gets too. Which in practice should rarely be an issue.

Biggest problem with the Duo Charge besides the price, yes it handles 30A, but if Starter asks for more than that, it doesn't just limit current or self-recover after a delay, it completely shuts down.

For DCDC chargers, I like Sterling's B2B units best, including the Promariner equivalents.

Charles says just a few months til the 180A version is released.

Yes, pricey.

As an example of all this, I have been underway for maybe 5 hrs. My house batteries are still at absorption voltage. My start battery took maybe 30-60 minutes to recharge completely with it's own charger. With an ACR or Echo Charger, it would still be getting charged at absorption voltage, way past when it reached full charge.

If it's flooded, no harm done, just have to top up more often.

Even the low end DCDC like CTEK costs more than I'd spend on a dedicated Starter.

Which these days is getting to be zero. . .

http://www.cruisersforum.com/forums/showthread.php?p=2411171

twistedtree said:

You uncovered a nasty noise issue with the Duo that I hadn't heard about before. It has what I would consider to be another nasty flaw; if the battery being charged wants to draw more than the Duo's rated 15A, rather than current limiting at 15A, the Duo shuts down. It restarts after a while, but will continue to shut down until the battery is charged enough to no longer draw more than15A. That means if the Duo is the only charge source, the battery will never get recharged. So when your battery is the lowest and needs the most charge, is exactly when the Duo can't charge it. How silly is that?

I'm not really a Mastervolt fan, but they have a DC to DC charger that works like the Duo, but current limits rather than shutting down. It's called the MAC something or other. Thats what I ended up using in this situation. The down side is that it's a bit pricey - around $400 if I recall. I think all the other approaches cost $200 or less.

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john61ct said:

Biggest problem with the Duo Charge besides the price, yes it handles 30A, but if Starter asks for more than that, it doesn't just limit current or self-recover after a delay, it completely shuts down.

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Yes, you both identified the primary weakness of the Duo Charger. It is not clearly spelled out in the manual that it will cut out if the current demand is higher than 30amps. This makes it less than ideal for a large bank that will be drawn down very far. The Duo Charge has been charging my 500Ah Thruster bank for a very long time with no issue. Partly is because, like a start battery, the bank is used for its high cranking amps but it not drawn down very far. When I got the boat, it had three Duo Chargers, one for genset battery, one for thruster bank, and one that had been used for the engine start battery. I guess I found out why the PO changed to the ACR for the start battery.

If you don't mind a start battery sharing the same charge voltage (up to 14.4) as your house bank, I think the Echo Charger is hard to beat for simplicity and price. It is only a tad over $100 and extremely easy to install.