Mixing new and old batteries

[Pizzazz]

Quick question. My house bank consists of four Interstate golf cart batteries. One pair is 18 months old, the other pair is 6 months old. The newer pair is wired further away, about 5 feet of extra cable.

When running heavy loads through the inverter, the older pair gives more current (40A) vs. the newer pair (30A) for a total of 70A. When charging, the older pair accepts more current (42A) vs. the newer pair (35A). The newer pair voltages are split exactly in the middle (say 6.16V under load) while the older pair voltages show a 0.15V differential.

Is this normal variation between batteries or is one of the older batteries damaged? Should I mix old and new in series or keep them the way they are? I usually take care of the batteries but when away from the marina I run them hard, often discharging to 30-40% SOC and I have let the old pair discharge completely about 2-3 times in the last 18 months (away from the boat, someone knocked the shore connection out, etc.).

Thank you, comments much appreciated.

SV Pizzazz

[john61ct]

Much better to replace them all at the same time.

[rom]

Those are quite new batteries anyway that should get along together. From what you describe the older batteries are actually healthier ... Well, IMHO it is a typical balancing issue. How are the 2x 12V sets connected to the boat ? Could it be that both the + and - are taken from the older pair ? (where you should take - from older and + from newer, or the other way round)

[NYSail]

How many cycles does a battery have to go through before it is at its top performance?? Maybe that is the issue here. The older ones have cycled and reached that point and the new ones are still breaking in..... I am actually calling Trogen today to ask that question for my 4 month old 6v t 125s (4). Somewhere I heard 20+/- cycles...... or is this a myth??

Greg

[sailorboy1]

Originally Posted by Pizzazz

Thank you, comments much appreciated.

SV Pizzazz

Are they wired correctly? Otherwise the shorter cable run battery will carry more of the load.

https://www.trojanbattery.com/tech-s...y-maintenance/

[john61ct]

Originally Posted by NYSail

or is this a myth??

Not a myth, but also impossible to quantify precisely in real-life conditions, too many variables.

Certainly within a month or two, and if not ideal coddling (getting back to 100% Full most cycles, low C-rate discharging, cool temperatures) the factors pulling capacity down, normal wear & tear of suboptimal usage,

may well overcome the "breaking in bump" - which is not that large an increase even in lab conditions - soon after the commissioning protocols have been followed.

From OP description that seems certain here, these batteries are long past "good as new", the unbalanced behaviour is inherent in paralleling strings, and the connection wiring is likely a major contributing factor.

[skipgundlach]

I'm facing replacement of one dead-cell battery and one perfectly good one in a pair that is approaching 3 years old. The other pair was replaced serially about 3 months apart, and is approaching a year old.

Trojan support (long story about having all 4 of the original set fail from a dead cell omitted) says that if the new pair isn't in series with the others, it will be fine, and the short time difference in the now-OK pair is uneventful to its health.

So, if you have a 12V pair newer than the other to which it's paralleled, it should not matter if it's a single bank. If it's not, then you have two different banks, and the performance difference is moot.

However, as covered in many different threads, having matching cables and lengths in parallel and series jumpers is significant. Yours sound unequal from the description...

[Pizzazz]

Thank you for the replies. As you suggested, it is a wiring problem. I swapped the battery locations to confirm that it was the length of the cable, not the battery condition that was giving me different currents. The voltage differential disappeared after cleaning the contacts.

The key learning is that if you have batteries in two different locations you need three heavy duty wires between the locations, not two in other to wire them correctly in a series/parallel configuration. Should have been obvious from the start.

SV Pizzazz

[ramblinrod]

Originally Posted by Pizzazz

Thank you for the replies. As you suggested, it is a wiring problem. I swapped the battery locations to confirm that it was the length of the cable, not the battery condition that was giving me different currents. The voltage differential disappeared after cleaning the contacts.

The key learning is that if you have batteries in two different locations you need three heavy duty wires between the locations, not two in other to wire them correctly in a series/parallel configuration. Should have been obvious from the start.

SV Pizzazz

Yes. Replacing a single battery in a bank is normally acceptable if the rest of the bank is in good state of health. If the batteries only lightly used and especially if lightly used, no problem whatsoever. It doesn’t make sense to recycle several batteries with 4 years life left, to get another 6 months from a new one.

Your issue is the load resistance between the batteries was not equal due to cable lengths. Ideally, each battery should be run to a pos and neg bus bar (or less preferential, power post) by equal lengths of cable, even if one bank must be some distance away from the rest. Yes it will cost a little more in cable, but it will save time, effort, frustration, and money in prolonged and more equalized battery life.