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Author Topic: Correct way to combine multiple batteries for backup and charging?  (Read 9150 times)
KC9IFF
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« on: July 05, 2011, 06:34:24 PM »

I have two 80 amp-hour AGM batteries.  Same manufacturer, same model.  I'd like to use both of these batteries in an automatic fail-over battery backup system in my station.  What is the best way to set up a system like this?

I could connect both batteries in parallel and call it a day.  However, I'm concerned with what'll happen if both batteries aren't identical or if one develops a bad cell.  So I'm considering getting a high-quality two-bank battery charger and connecting one charger bank to each battery.  Doing that would require not connecting the batteries in parallel, but I assume if I do that there'd be no way to discharge both batteries.  And if they are in parallel for discharging, I'd assuming the bad cell scenario would affect discharging just as it affects charging, right?

So, is there a device or devices that will allow two batteries to be charged independently when A/C power is present, and some sort of combiner that'll combine them during discharge?  Or should I keep it simple and connect them in parallel?

Because this system includes some automated devices, I'd also like to integrate low voltage disconnects to prevent over-discharging the batteries during an extended outage.  How would that fit into the mix?  Will it work okay with one LVD device connected at some point after both batteries are combined, or do I need a separate LVD for each battery?

Also, does anyone have a recommendation for a fail-over device like the PowerGate that does not include a built in battery charger?  Ideally one that can handle more than 40 amps.

Thank you,

Luke
KC9IFF
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K1CJS
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« Reply #1 on: July 05, 2011, 06:49:45 PM »

The easiest way to determine if the batteries are OK is to connect them in parallel and charge them until the charger indicates full charge.  Then disconnect them from the charger and each other.  Both should read nearly identical as far as voltage goes.  If one is significantly different, you know right away that there is an issue with one of them.

Now let them sit for a few days.  Again read the voltage.  If both were nearly identical, they should still be nearly identical if they're both good.  In that case, there is no problem connecting them in parallel, both to a single charger and drawing on both for power.  Check them separately every six months or so, to make sure that they're staying up to snuff.

I had a bank of twenty 12 volt 7.5 amp hour batteries (SLA type) set up to run my constant power supply for my computer systems, in two parallel banks of ten in series to supply 24 volts.  Those batteries were used, out of fire alarm panels when the batteries were replaced after 3 years.  They ran my setup well for over three more years before they started to give up the ghost.  I just replaced them all recently.
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W8JX
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« Reply #2 on: July 05, 2011, 07:33:19 PM »

I actually had a aux battery in my SUV have a cell short out recently and it did not cause catastrophic damage but it did discharge main battery down to 10.5v and of course it would not start. Been using multi and dual battery setups in SUVs for over 20 years and this was first failure like this. Vehicle is not driven daily and may sit for a few weeks at a time between usage. Normally I isolated batteries when it sits for a while but did not this time.

Getting back to question if you are really worried about this and want up to a 40 amp discharge rate from pair a simple solution is you could place a 20 to 25 amp fuse (or circuit breaker) between negative terminals of paralleled batteries and it opens if one battery shorts so it will not kill other. And as far as checking voltage of each battery, this is not a big concern as a little difference in voltage is not a deal killer here.
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K1CJS
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« Reply #3 on: July 06, 2011, 04:38:23 AM »

You're correct in saying that that isn't a big concern here, John, but I was just describing a way to check the batteries for cells that were going bad, nothing more.  Usually, hams obtain these batteries used, not new, and what I described is the easiest way to check them to see if they're still in fairly decent shape.
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W8JX
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« Reply #4 on: July 06, 2011, 06:10:31 AM »

You're correct in saying that that isn't a big concern here, John, but I was just describing a way to check the batteries for cells that were going bad, nothing more.  Usually, hams obtain these batteries used, not new, and what I described is the easiest way to check them to see if they're still in fairly decent shape.

I can see your point but a voltage check will not reliably show if a battery has reduced capacity from age unless you load test each one too. I have several UPS here that use dual 8 or 9 amp gel cells (same size as 7 amp ones) for 24 volts and one that uses six 18amp cells in series parallel for 24v. (1000 watt fan cooled UPS with 2 batteries on board and 4 more in a external case that are fused as above) When a battery goes bad I replace the pair with new ones as they can be reasonable had on line. I would suggest that a better way to check condition of multiple batteries is series parallel would be to check voltage of cell after a discharge cycle of pack. If some are much lower than others they have reached the end of their service life. 
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K1CJS
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« Reply #5 on: July 06, 2011, 06:46:30 PM »

Yes, it can also be done that way.  The method I mentioned does rely on batteries that are used, but not at the end of their useful life.  Hospitals and nursing centers have to replace batteries after a set time period, and the discards are usually still good.

Usually, SLA batteries will not hold a full charge when they're at the end of their useful life.  They may hold voltage level, but will drop off fast once they are connected to a load.  That is usually the way you know you better ante up for new ones.
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W2EM
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« Reply #6 on: August 06, 2011, 08:16:04 PM »

You can use a MFJ BATTERY VOLTAGE BOOSTER to turn off the power when the battery get to a predetermined level. It can be set to shut off at 9, 20 or 11 volts.

http://www.mfjenterprises.com/Product.php?productid=MFJ-4416B
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W8JX
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« Reply #7 on: August 06, 2011, 09:09:33 PM »

You can use a MFJ BATTERY VOLTAGE BOOSTER to turn off the power when the battery get to a predetermined level. It can be set to shut off at 9, 20 or 11 volts.

http://www.mfjenterprises.com/Product.php?productid=MFJ-4416B

Problem with a booster is that it adds more drain on batteries  (shorter life) to run it.
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KD4LLA
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« Reply #8 on: August 28, 2011, 02:05:39 PM »

Instead of using two batteries I would use one battery of a bigger size.  The trouble with multiple batteries is one will go bad before the other and there will be little warning.  I get batteries (free, as in free beer) that have been cycled from hospital genset duty.  They are about 18" x 12" x 12".  They last for a year to two years and by then I get another one.  I keep it charged by using a solar panel...

Mike
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AA4PB
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« Reply #9 on: August 28, 2011, 03:35:09 PM »

AGM batteries are connected in parallel all the time - its a standard way of combining batteries for increased capacity. You'll find large computer UPS units, fork lifts, and golf carts all using parallel 12V batteries.
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W8JX
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« Reply #10 on: August 28, 2011, 04:36:57 PM »

AGM batteries are connected in parallel all the time - its a standard way of combining batteries for increased capacity. You'll find large computer UPS units, fork lifts, and golf carts all using parallel 12V batteries.


Actually large UPS units, electric fork lifts and golf carts use more than 12v. Serious UPS units and inverters are at least 24 volts and use 12v batteries in series parallel.  Electric forklifts usually have a single large battery that can weigh a few tons in a big unit and usually at least 42 volts too. They use the single large battery in a design that will actually allow then to swap them out if a few minutes with a fresh if need be. On golf carts, every one I have looked under the hood on uses 6 volt deep cycle batteries in series parallel for 42v or more. It is not practical to power them with 12v because of cable size and connectors that would be required for amperage vs using 42 volts or more.  It is just a question of time before they even move cars off 12 v systems and their limitations too.
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W5CPT
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« Reply #11 on: August 29, 2011, 11:22:17 AM »

You can get a battery isolator at an RV shop.  It allows you to hook up two batteries, use them both, charge them at the same time but if one goes dead it keeps the other from discharging through the bad one.  They can also be had through Northern Tool for less than $30 <- cheap insurance.

Clint - W5CPT -
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