question

geoffrey-rathbun avatar image
geoffrey-rathbun asked

Mixing New Battery with One Year Old Lead-Carbon Battery

I have a four battery Narada serial battery string giving 24V, which is 11 months old.
I want to add four more batteries to the string to make it a 48V setup.
Someone told me you should not mix old batteries with new ones.
(But, how do you define an "old" battery ? Does one or two cycles make it "old" ?)
I don't want to sell the existing batteries and buy 8 new ones, I only want to buy 4 more.
Does anyone have any advice for me ?


However, my existing batteries are still fairly new, and have only undergone, I guess about this history of discharge:
200 days at 15% DoD
50 days at 30% DoD
50 days at 50% DoD
30 days at 75% DoD
My feeling is that my existing batteries are quite close in condition to new batteries, and so adding another 4 new ones will not disturb them all too much.

ALSO, if I equalise the "old" batteries before I add the new ones, will that help ?

battery chargingbattery
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3 Answers
JohnC avatar image
JohnC answered ·

Hi @Geoffrey Rathbun

There's probably no real answer for this. All you can do is try it.

In a series string the difference between the new and old will be noticeable in the deviation of the mid-point Voltage from 'half-way' (this can be monitored by one of the Victron shunt devices, or multimeter). This might be fine, but even if it does deviate it may well improve over time, as many new batts won't develop full capacity until after quite a few charge cycles.

I wouldn't worry about equalizing them first, but I'd only connect them up with both fully charged. I'd also back off the level of the Absorb charge to avoid anything aggressive, and also to get below what half the pack might experience. Yes, I'd have the newies on one side of the midpoint.

Perhaps even back off the max charge amps too until you gain some confidence. And if you notice a difference in batt temperature between old and new you have a real problem.

Good luck.

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kevgermany avatar image
kevgermany answered ·

You're probably well into the service life of the existing set.

Did you consider rewiring the existing set to 48V, then moving to an 8 battery set when these are dead?

Won't give a capacity increase, however.

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geoffrey-rathbun avatar image
geoffrey-rathbun answered ·

Thanks for answer. Unfortunately cannot modify existing to 48V as current set is maxed out to 24V.

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