Batteries of different capacities wired in parallel

[lw395]

A lot of what he says about electricity makes sense though, I couldn't do without his bible: http://amzn.to/1g7B7gH

I think if this scenario is going to cause a problem would depend on the batteries, and the way they are wired. I would make sure to wire them with the charge/discharge cables at opposite ends, and use large capacity cable for the interconnects.

Fair comment, but I think it also depends on how the battery bank is to be used, and what you consider to be 'a problem'.
Also what does that 'problem' cost you over the lifetime of yacht ownership, vs what does it cost to avoid the problem.

Some battery banks in industry get used much more heavily than a yacht's batteries.
For instance, think about the batteries in an uninterruptible power supply, which are designed to get flattened in just a few minutes before the generator kicks in. The consequences of those not sharing properly could be expensive beyond the value of the batteries.
I suspect the rule is good guidance if you want to parallel starter batteries, but probably just a nicety for most lightly used house batteries.
House batteries that get spanked by heavy loads like big inverters might be a grey area?

 

[CAPTAIN FANTASTIC]

Yes you can "mix and match" 12v batteries of similar capacities in parallel and charge them as normal. It does not really matter whether they are starting batteries or leisure batteries.

The charge will flow throughout and the good batteries will hold the charge the bad batteries will not, some batteries will have more resistance than others; batteries which can not hold the charge for long when tested individually can be thrown away.

However, there is no need to get so technical about it; keep things simple, it is only a sailing boat not a spacecraft.

 

[pvb]

You are overcharging. Simple as that!

Maintenance-free lead-calcium batteries are remarkably resilient to higher charge voltages. I had an Adverc booster on my last boat, which delivered 14.8v or so to the batteries fairly constantly. The batteries were maintenance-free, and routinely lasted about 8 years, which isn't too bad, I reckon.

 

[Bru]

Yes you can "mix and match" 12v batteries of similar capacities in parallel and charge them as normal. It does not really matter whether they are starting batteries or leisure batteries.

The charge will flow throughout and the good batteries will hold the charge the bad batteries will not, some batteries will have more resistance than others; batteries which can not hold the charge for long when tested individually can be thrown away.

However, there is no need to get so technical about it; keep things simple, it is only a sailing boat not a spacecraft.

I debated just ignoring this but I can't! It takes the prize as quite probably the worst and most inacurate piece of bad advice I've ever seen in these forums

What you say was true (for a given value of true) and I suppose could be said to still be true (ditto) for basic flooded cell (aka open) lead acid batteries

But times have seriously moved on in battery technology. Sealed batteries, AGM batteries and so on need to be charged differently and it's not just the voltage that matters

Yes, OK, you won't have an explosion and you will get a usable battery bank - for a while - but if you mix and match different battery types (or even different makes of sealed or AGM batteries) you will almost certainly reduce the effective capacity of the battery bank and, critically, shorten (possibly dramatically) the service life of the batteries

And there are potential safety issues too. Charge sealed lead acid or AGM batteries at the default rate for flooded batteries and you'll cook them. Probably not badly enough to cause a major issue but it can happen. (That's why the latest battery charging systems have temperature sensing)

Yes, it's only a sailing boat. And modern sailing boats are increasingly sophisticated beasties

 

[CAPTAIN FANTASTIC]

I debated just ignoring this but I can't! It takes the prize as quite probably the worst and most inacurate piece of bad advice I've ever seen in these forums

What you say was true (for a given value of true) and I suppose could be said to still be true (ditto) for basic flooded cell (aka open) lead acid batteries

But times have seriously moved on in battery technology. Sealed batteries, AGM batteries and so on need to be charged differently and it's not just the voltage that matters

Yes, OK, you won't have an explosion and you will get a usable battery bank - for a while - but if you mix and match different battery types (or even different makes of sealed or AGM batteries) you will almost certainly reduce the effective capacity of the battery bank and, critically, shorten (possibly dramatically) the service life of the batteries

And there are potential safety issues too. Charge sealed lead acid or AGM batteries at the default rate for flooded batteries and you'll cook them. Probably not badly enough to cause a major issue but it can happen. (That's why the latest battery charging systems have temperature sensing)

Yes, it's only a sailing boat. And modern sailing boats are increasingly sophisticated beasties

Yes, I was referring to lead acid batteries

 

[Bru]

Yes, I was referring to lead acid batteries

And, unfortunatley, still getting it wrong

Open / Flooded? Sealed?

And clearly not understanding that gel batteries (AGM etc.) are lead acid batteries

And then I could start in about the issues of combining starting batteries, with traction batteries due to the different charging profiles that the different types require for proper performance and adequate service life

Yes, it is true, that in days of old when men were bold we just chucked a couple of old car batteries on the boat, plugged them in to the alternator output and once in a while to a very crude mains charger but if you're investing hundreds of quid in new batteries, and a single decent new battery is going to cost getting on for a ton, it pays to get it right

 

[JumbleDuck]

Why the need to be the same chemically?

For discharge it probably doesn't, much, but chargers seem to care what they are feeding, so it probably makes sense to stick to one type when they are going to be parallel.