Tired Old meets Fresh New Battery

[elhix]

I've got a tired old 150a/hr battery aboard (sealed lead deep cycle) it saw action for two summer but unfortunately took a hit (accidentally discharged to <11v) during the first summer after it was never quite the same again. It still functioned well enough but there was no doubt it had lost some capacity.

I've got two new batteries of the exact same model to install next week, my question is: can I incorporate the old, tired one into a bank with the two shiny and new batteries??

Any input appreciated!

[ranger58sb]

No.

But if you have physical space for a third battery and need additional Ah, now's a good time to get another new one.

-Chris

[sailorboy1]

You can, if you want your nice new batteries to be dragged down by your old one.

That's my way of saying that I wouldn't do it.

[CharlieJ]

Don't do it.

[elhix]

Skip it is then... Thanks!

[Andina Marie]

Although the answers you've received are very common and pervasive in bulletin boards they are wrong. Their advise would be correct for batteries in SERIES but it does not apply to batteries in PARALLEL.

Each battery will contribute to the whole capacity in proportion to its own capacity. The OLD one can't "run the new ones down". For one battery to run another one down it would have to be at a different voltage. While it is true that when you first connect there will be voltage differences and current will flow for a few seconds but then they will then all be at the SAME voltage. If at the same voltage no current flows between the batteries.

When connecting in parallel you can mix different size, different age, even different chemistry except that you may have to set your charger to suit the most critical battery, for example a lead-acid in parallel with a sealed gel battery should be charged at the gel battery rate, not the lead acid rate, so charging may take a few minutes longer but they will still all get a full charge.

It is a common misconception that the current divides equally between batteries on charge or when supporting a load and that 33% would be going to (or coming from) each battery but this is totally wrong. Each battery has a different internal impedance and the current divides exactly in proportion to their capacity. The old tired battery may be only supplying 10% of the load instead of 33% but it is still supplying current to the load and doing its share.

[charliehows]

Originally Posted by Andina Marie

Although the answers you've received are very common and pervasive in bulletin boards they are wrong. Their advise would be correct for batteries in SERIES but it does not apply to batteries in PARALLEL.

Each battery will contribute to the whole capacity in proportion to its own capacity. The OLD one can't "run the new ones down". For one battery to run another one down it would have to be at a different voltage. While it is true that when you first connect there will be voltage differences and current will flow for a few seconds but then they will then all be at the SAME voltage. If at the same voltage no current flows between the batteries.

When connecting in parallel you can mix different size, different age, even different chemistry except that you may have to set your charger to suit the most critical battery, for example a lead-acid in parallel with a sealed gel battery should be charged at the gel battery rate, not the lead acid rate, so charging may take a few minutes longer but they will still all get a full charge.

It is a common misconception that the current divides equally between batteries on charge or when supporting a load and that 33% would be going to (or coming from) each battery but this is totally wrong. Each battery has a different internal impedance and the current divides exactly in proportion to their capacity. The old tired battery may be only supplying 10% of the load instead of 33% but it is still supplying current to the load and doing its share.

thanks for that information - I've been cycling batteries through my system for years and have never seen this 'dragging down effect' but i've always been a little nervous about it - so now i can relax - and feel good about getting a full, green as possible use from a product most people would have thrown away a lot earlier.

[deckofficer]

I'm part of the "common and pervasive" group looking to learn something. If I understand this right, an old battery that only delivers after charging 20% of its new capacity is in parallel with a new battery, their voltage's stay in step during discharge loads? If the current draw for the load is say 10 amps, then the new battery provides 8 amps and the old battery supplies 2 amps?

[Andina Marie]

Originally Posted by deckofficer

I'm part of the "common and pervasive" group looking to learn something. If I understand this right, an old battery that only delivers after charging 20% of its new capacity is in parallel with a new battery, their voltage's stay in step during discharge loads? If the current draw for the load is say 10 amps, then the new battery provides 8 amps and the old battery supplies 2 amps?

Yes their voltage MUST stay in step during discharge (or charge) because they are connected together, they HAVE to be at the same voltage. So if discharging into a load and the voltage drops (on both) from 12.8 to 12.7 then the "new" battery will provide the amount of amp-hours equivalent to that voltage reduction and the "old" one does its smaller share. When a battery gets "old" its internal resistance goes up. With a higher internal resistance, the current it can supply (or absorb) is reduced in proportion until it gets too old, for example, to be able to produce enough current to start an engine.

[deckofficer]

Question 2 would be the issue of self discharge. Older cells self discharge at a faster rate so wouldn't the propping up of these cells with the new ones in parallel cause the new ones to age quicker?

[noelex 77]

Mixing old and new batteries in the same battery bank is not generally recommended for good reason.

The most serious problem is if the old battery drops a cell. In this case the the old battery will discharge the new battery to to a dangerously low voltage. Causing permanent damage to the new battery.
It is very difficult to detect the old battery deteriorating when it is connected in parallel with a new battery, so there a few warning signs.

It is a shame to throw away an old battery that still has some capacity left in it.

With a long distance cruising boat having two separate house battery banks is a good idea.
It allows safe utilisation of an old battery. In the absence of such an arrangement. I would recommend new batteries.

[motion30]

Have you tried to equalize the old battery

[Andina Marie]

Older batteries may appear to have a higher self discharge rate but that is because they have less capacity so the symptoms show up more than with a new battery. Measurements on current from battery to battery are typically less than a couple of milliamps so under worst case conditions the old battery would steal 1% of the good battery every month which is small compared to the good battery's own self discharge rate.

Cell failure virtually never occurs on an idle battery or under light load conditions. It will usually happen when charging and occasionally under heavy load such as starting an engine. Under both conditions the cell failure should be obvious.

Cell failure is a possibility for all batteries in parallel and not limited to "old" batteries.

[kentobin]

It always amazes me to what lengths people will go to to save a few bucks, even myself.

If you want a science experiment keep the old battery.

I used to work on auto electrics and I can't ever remember a cell going out on a new battery. Old batteries yes but I have to admit even that didn't happen very often.

[Wissing]

Originally Posted by Andina Marie

Although the answers you've received are very common and pervasive in bulletin boards they are wrong. Their advise would be correct for batteries in SERIES but it does not apply to batteries in PARALLEL.

Each battery will contribute to the whole capacity in proportion to its own capacity. The OLD one can't "run the new ones down". For one battery to run another one down it would have to be at a different voltage. While it is true that when you first connect there will be voltage differences and current will flow for a few seconds but then they will then all be at the SAME voltage. If at the same voltage no current flows between the batteries.

When connecting in parallel you can mix different size, different age, even different chemistry except that you may have to set your charger to suit the most critical battery, for example a lead-acid in parallel with a sealed gel battery should be charged at the gel battery rate, not the lead acid rate, so charging may take a few minutes longer but they will still all get a full charge.

It is a common misconception that the current divides equally between batteries on charge or when supporting a load and that 33% would be going to (or coming from) each battery but this is totally wrong. Each battery has a different internal impedance and the current divides exactly in proportion to their capacity. The old tired battery may be only supplying 10% of the load instead of 33% but it is still supplying current to the load and doing its share.

Nice to see somebody who knows his stuff.