UPS / Battery Question

Question

I have two Cyberpower 1400va (900 watt) pure sine wave UPS systems. They are over 5 years old and the lead acid batteries inside them are shot. The units use two 12v 8AH SLA batteries in series and for what they cost to replace I could almost replace the entire UPS units. I live in Hawaii so the shipping cost of lead acid batteries is often higher than the cost of the batteries themselves and Amazon won't ship them here for free prime or otherwise.

My idea is to buy two 12v SLA batteries off eBay from a manufacturer I have been buying the same batteries from for 15 years. They've solved the shipping cost dilemma by designing the batteries to fit into flat-rate boxes. They are 18AH each so obviously won't fit inside the UPS units, but that is not a problem for my application.

These UPS units aren't used in a traditional manner of being backup power. I have a DIY solar setup that uses a transfer switch to switch loads back and forth from solar and grid power based on the voltage of the solar battery bank. The UPS systems are between the inverter and the loads to take the abuse of the power transfers. The UPS batteries don't usually get cycled because it's a rare occurrence the grid goes out at the same time the solar battery bank is depleted, which is probably why I got 5 years out of the original batteries. The UPS units use an Automatic Voltage Regulation (AVR) in line interactive UPS system which stabilizes the incoming AC signal to maintain output power at a nominal 120 volts by controlling high and low voltages without resorting to battery power, but when the batteries are shot the UPS units go into a protected mode and power off if the AVR is overwhelmed.

My question is, do I even need to buy four batteries, or could I use two batteries and connect them to both UPS units? I understand the batteries will be receiving the charging current from both UPS units, but Cyberpower lists the recharge time at 8 hours, so the charging current doesn't seem like it would be too high for these batteries. The other issue is that if both UPS units start drawing from the batteries at the same time, the current draw could theoretically be higher than what the batteries can deliver for any length of time, but that is easily remedied by limiting the loads of both UPS systems to a max of 900 watts (combined) which each UPS is individually rated for.

Thoughts?

Answer 1

I think you're seriously looking for trouble if you try to use one set of batteries in two UPSes at the same time. In theory, discharge will be OK - the batteries will be supplying both loads at the same time, which is no big deal. But I think charging will be messed up. Both UPSes will be trying to charge at the same time and interact with each other. I'm not quite sure what would happen, but I have a feeling you will end up with one or more of:

• One UPS zaps the other.
• Both UPSes charge at the same time and charge the batteries too much.
• Both UPSes charge at the same time and don't charge enough because the double charge makes it look like the batteries are at a higher charge level than they actually are.

I just can't see it working well.

What I would recommend is to get 4 batteries and do it right - 2 in each UPS. Oversize batteries, as long as they are the same chemistry (and therefore the same charging regimen) should be fine.

Alternatively, get a new UPS that is designed to handle external batteries. Even if those external batteries are normally provided by the UPS manufacturer (i.e., a module that includes batteries and cables and a case that matches the main unit), it should be able to handle more/larger batteries, again as long as the chemistry is the same.

Answer 2

The units use two 12v 8AH SLA batteries in series and for what they cost to replace I could almost replace the entire UPS units. I live in Hawaii so the shipping cost of lead acid batteries is often higher than the cost of the batteries themselves

That's completely normal (on the mainland also) for UPS's and jumpstarters. Countless numbers of those units go into the waste stream because it's cheaper to buy a new one than buy just the battery component. This is a side-effect of commodity production and distribution.

They've solved the shipping cost dilemma by designing the batteries to fit into flat-rate boxes. They are 18AH each so obviously won't fit inside the UPS units, but that is not a problem for my application.

OK, you're good with the Frankenstein. What has me concerned is the UPS's charging circuit may not be ready for a larger battery.

The UPS systems are between the inverter and the loads to take the abuse of the power transfers. The UPS batteries don't usually get cycled because it's a rare occurrence the grid goes out at the same time the solar battery bank is depleted, which is probably why I got 5 years out of the original batteries.

Well then, you probably don't need quite as large batteries. I'm sure your supplier sells a variety, I would go for ones that fit inside the unit (even with some slop) and will still ship inside that fixed-rate box.

My question is, do I even need to buy four batteries, or could I use two batteries and connect them to both UPS units?

My concern is distance. Are these units right next to each other? If not, moving 24 volts at such power levels is not to be underestimated. With inverter losses, 900 watts is 50 amps at 24 volts. Voltage drop is very punishing at such a low voltage.

I'm not so worried about the two chargers fighting; not least, you could simply disable one of them, or disable both and use external charging.

Come to it, do they have golf carts in Hawaii? Because four golf cart batteries is 24V, and a golf cart battery is officially "dead" when it can no longer make it 18 holes + to and from the owner's house. That's a lot of life still in it.

Answer 3

I say go for it. Re discharge, figure the worst case discharge rate and make sure the single set of batteries can handle both UPSes at once. As you suggested, if you can limit the load on each UPS, that's a solution.

Re charge, there's no harm in using larger batteries than originally designed (assuming the same voltage!); it'll just take longer to recharge after an outage. I don't think one UPS charger will "zap" the other - the charging circuit is designed to handle a battery at anywhere from ~1.6 to ~2.5 volts per cell (about 10 to 15 volts for "12 volt" battery). It doesn't really matter if some of the voltage is coming from the other UPS or from the battery.

I've had good luck attaching old car batteries to tiny UPSes that were designed for 5 AH batteries - they take a while to charge, but if you don't mind, it works fine.

You might consider using LiFePO4 batteries instead of lead-acid. They'll last a lot longer (but do cost more).

Finally, keep the distance (lead length) short from the battery to the UPSes - there is indeed a lot of resistance loss at 12 or 24 volts. Use really thick wire if you need to run any distance.