SMX1500RM2U w/ 1 SMX48RMBP2U ... additional packs increase charging rate?

[krep_apc]

Hello,

I have 1 SMX1500RM2U and 1 SMX48RMBP2U external battery pack, with new  batteries. Both are working ok, except the maximum charge rate going into the external battery pack is about 80W. At ~54 V for 4 series batteries, this amounts to about 1.5 Amps charge rate, which is way, way below the safe charge rate for these batteries. The SMX1500RM2U is able to detect the external battery pack and shows "1 external battery pack" installed. Would adding additional packs cause the SMX1500RM2U to increase it's charging rate? Or it just puts out the same amperage no matter how many external batteries are connected and adding more batteries will just cause the entire system to take longer and longer to charge?

Last, would changing the V constant on the battery to a slightly higher or lower setpoint cause the UPS to charge at a higher wattage?

Thanks for any help you can provide!

-Noah

[krep_apc]

Answering my own question:

Increasing battery packs does not seem to increase the SMX1500RM2U output to the batteries for charging purposes. I increased the detected battery packs to 3 instead of 1, but the power to the 1 external battery connected remained approximately the same. It appears the SMX1500RM2U outputs a fixed "Current controlled" charge rate of ~1.5 Amps. This has the net result that, as the batteries are depleted and voltage drops, the charger actually puts out less total power over a given amount of time. Adding more battery packs, even though the SMX1500RM2U detects the additional packs, does not appreciably increase the charge rate. I'm not sure why APC designed the unit this way, I guess for simplicity and lead acid battery life, since I had read some topology description for "line interactive" UPS that in order to charge the battery, the UPS simply runs the inverter in reverse as a battery charger, which would lead one to think the max charge rate would be limited by the UPS's maximum wattage rating. I wonder if it could be upped via software...

-Noah

[Brad_C · Accepted solution]

From what I can tell, the SMX series uses roughly the same charging circuit as the older generation SmartUPS. Every SmartUPS I've looked at has been built to charge the internal battery to ~90%  in ~180 minutes. There has never been any variation to charge current. It was just accepted that more packs equalled a longer recharge time. 80W does seem low though.

The 1500 is a 48V UPS, and from what I can tell has an internal 9Ah battery. That'd require ~3A to get to 90% in 180 mins. So if you put external batteries on you'll see a consequential division in current. The SMX48RMBP2U has 18AH of battery, so in *theory* it'd be divided in a 1/3rd / 2/3rd ratio, but that's in real world ideal conditions.

How did you measure the charge current?

Oh, and can you increase the charge current in software? No.