replacing ups lead acid battery with lifepo4

i've seen a few threads about this on this forum but i still have one question
i have an older apc smartups 760
it has an external anderson connector
i've been using a 35ah sla
i bought a 100ah 12 volt lifepo4
would it be feasible to add a diode to prevent the ups from charging the battery
and add a dedicated lifepo4 charger

fxkl47BF said:

i've seen a few threads about this on this forum but i still have one question
i have an older apc smartups 760
it has an external anderson connector
i've been using a 35ah sla
i bought a 100ah 12 volt lifepo4
would it be feasible to add a diode to prevent the ups from charging the battery
and add a dedicated lifepo4 charger

Click to expand...

assuming 760 is 760VA

760/12.8/.8 = 74A

The 100Ah 12.8v LFP should be able to handle this.

Any diode will produce voltage drop and must be rated for the forward current and corresponding power.

Most UPS seem to charge to a float voltage of 13.8V. This is a little on the high side for LFP, but it might work. 13.6 would be better.

I have a number of older APC smartups and those numbers for the battery do not seem to jive correctly

The 760 takes a 12v9ah battery originally from what I can find.

And most of APC's line won't turn on without a sucessful selftest and battery check.

So your diode to prevent charging is a good idea as long as it doesn't interfer with the self test. Running without a diode would be a problem as well since the charge circuit won't put out the correct voltage for the actual charge and would overheat and let the magic smoke out when the charger had to run for 2 million years to charge the batteries up.

IMO you are better off junking the UPS and getting an inverter with a 5ms switchover + charger and just run from battery all the time and the inverter will keep the battery topped up when needed. Hook a SCC up and you can have solar.

I use one of these to power my server and desktop ups from a 12v200ah battery.


I switched when the battery pack in my smartups 2200 was due to be replaced again after only 2 years. It takes 2 x 24v 18ah packs in series (48v total and the charge circuit puts out aroung 54.5v) and they are not cheap to buy or ship or dispose of.

If you want to stick with APC smartup I have several old 900s and larger you can have for the cost of shipping.

I agree with forgoing the idea. As an alternative to a switch (wherever that may be sold and what its name is?), a charger+inverter has a switch in it already to pull from AC or battery.

I run purely off batteries w/out a UPS or switch. Its pretty reliable and more reliable than a generator because I've let the generator run out of gas and oil and then it drops the load. With the battery I just need to be sure I have enough juice for the time I want.

robbob2112 said:

If you want to stick with APC smartup I have several old 900s and larger you can have for the cost of shipping.

Click to expand...

i would like to get them but the shipping for heavy items to the gulf coast would cost more than new equipment
i'll use what i've got until it dies
by then who knows what will be available

There are some lifepo4 batteries advertised as replacement for apc ups. Lifepo4 can charge at 14.5, and float at 13.5v.

I have at 48v to 12v converter and run it directly from my battery. The converter blocks the reverse voltage. The back-ups 350 complains but runs fine.

I bought a 1.6kw Powmr off-grid AiO for this reason cost about $150 for the AiO and grabbed two 380w panels for the heck of it now not only does my server Rack have a far bigger offline capacity but it runs for free during the day.