APC smart UPS 750vA which BMS?

[ismail gul]

I am planning to replace my APC smart-UPS 750vA battery with lithium ion batteries (18650). right now it has lead acid battery 2x 7ah each connected in series.
i have built a 7s10p battery pack i have bms 24v 25A rated ordered from aliexpress. my question is will it work for my project ? can this bms provide enough power for my project?? please help

bms link is here

2.99US $ 68% OFF|Balanced Bms 7s 24v 15a 20a 25a Lithium 18650 Battery Charge Board Equalizer With Ntc Temperature Protection For Ebike/escooter - Battery Accessories & Charger Accessories - AliExpress

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[OffGridInTheCity]

750va is aprox 750w, so that's 750w/24v battery = 31a max draw from the battery. I'd suggest going up to a 50a BMS for a little headroom over 31a max. The largest option for the BMS above is 25a - doesn't support enough current flow.

 

[ismail gul]

you are right but i have checked its specification on its official websites it shows 500W that is why i m confused can you please check it see if that is UPS's actual wattage.

 

[OffGridInTheCity]

you are right but i have checked its specification on its official websites it shows 500W that is why i m confused can you please check it see if that is UPS's actual wattage.

You can google the specs of your APC. I have several APC 1500 and APC 3000(s) and the number in the name is typically the max watts it can handle. So 750 is typically 750w peak.

 

[ismail gul]

i have attached image of its manufacturer features please take a look.

 

[Korishan]

The number in the APC is the Volt Amps rating, not watts. So the APC750 is not 750W.

Well, sort of. If the loads on the UPS are 100% power factor, then yes, it'd be 750W. However, the majority of the loads is going to fall somewhere between 60 and 75% power factor. So essentially, the 750VA rating will actually be about 450W - 560W range.
I ran into this issue when I was sizing a UPS for the computer and had to find out the PF of my computer PSU, which is around 85-90%, depending on load.

 

[Korishan]

Addendum. I replied before reading the latest comment that had the included the datasheet image of the UPS. So that agrees with my assumptions

Another thing to note is that the BMS really just needs to be able to handle the voltage and the current the APC would be pulling under max load.
So with the assumption the UPS would be running at 100%, which is 500W, then 500 / 27V (which is about nominal) = 18.5A. As long as the BMS can handle at least 20A, you should be fine. If you can get a 25A rated one, that'd be even better.

 

[Halo.wings]

You mentioned that you had several UPS systems as well. Have you had any problems with the UPS systems not wanting to charge your batteries?

I am trying to build out a battery for a APC UPS SmartUPS 1500 - I have a 25A BMS on my batteries (not sure of the make or model but they look like the LTO 66160 Lithium Titanate battery but are Lithium Ion (out of an electric snowmobile that was being developed).

Anyway, each battery is around 50 Amp Hours - and I have them hooked up to the UPS. The UPS runs fine from the batteries but refuses to charge the batteries back up (just beeps that the batteries need to be replaced). I assume that there is something wrong with the BMS where it is not sending the correct signals to the USP to start the charging process.

If the batteries are fully charged (29.4V) then the UPS does not beep, but once they drop down to around 24V the UPS will just beep and not charge at all.

The batteries charge just fine from B6 battery charger or bench power supply (though the BMS but that is at max 2A).

So my question is there a BMS that works better with these APC UPS's or is it just hit and miss and hope I find something that works?

 

[OffGridInTheCity]

You mentioned that you had several UPS systems as well. Have you had any problems with the UPS systems not wanting to charge your batteries?

I am trying to build out a battery for a APC UPS SmartUPS 1500 - I have a 25A BMS on my batteries (not sure of the make or model but they look like the LTO 66160 Lithium Titanate battery but are Lithium Ion (out of an electric snowmobile that was being developed).

Anyway, each battery is around 50 Amp Hours - and I have them hooked up to the UPS. The UPS runs fine from the batteries but refuses to charge the batteries back up (just beeps that the batteries need to be replaced). I assume that there is something wrong with the BMS where it is not sending the correct signals to the USP to start the charging process.

If the batteries are fully charged (29.4V) then the UPS does not beep, but once they drop down to around 24V the UPS will just beep and not charge at all.

The batteries charge just fine from B6 battery charger or bench power supply (though the BMS but that is at max 2A).

So my question is there a BMS that works better with these APC UPS's or is it just hit and miss and hope I find something that works?

I have several APC UPS 1500s with 7s7p lithium-ion and they charge at 2-3a (50w-75w) with no problem. The APCs don't care about the type of battery or BMS except that charge voltage has to flow. Mine charge up to 27.6v +/- a bit. You're (24v nominal) battery would have to be <27.6 for charging to take place.

I would make sure you battery is 26v or lower and then look at the BMS. For example, can you temporarily bypass the BMS and hook the battery directly to the APC and see if it works. This would prove it's a BMS issue.

 

[Halo.wings]

I have several APC UPS 1500s with 7s7p lithium-ion and they charge at 2-3a (50w-75w) with no problem. The APCs don't care about the type of battery or BMS except that charge voltage has to flow. Mine charge up to 27.6v +/- a bit. You're (24v nominal) battery would have to be <27.6 for charging to take place.

I would make sure you battery is 26v or lower and then look at the BMS. For example, can you temporarily bypass the BMS and hook the battery directly to the APC and see if it works. This would prove it's a BMS issue.

After some checking it seems that the BMS is fine - The system works fine with or without the BMS.

What I am seeing now is the UPS reporting that the battery needs to be replaced (but just unplugging the UPS, the UPS reacts as expected, so I do not think there is a problem).

Do you leave the beeper attached to the UPS board or do you remove it? I am thinking of replacing it with a LED (in case there still needs to be a load there - Still need to research this).

On another note have you tried any of these cases that you can purchase from Aliexpress and put in 3S7P battery packs (12.6V)? I am trying them but not sure if they will provide enough runtime (still testing this).

I know these are LiFePO4 but that was my error, and I had to order different BMS units for them.

https://www.aliexpress.com/item/1005003405741082.html?spm=a2g0o.productlist.0.0.1ca35737V0AdN4&algo_pvid=972b6314-ecbe-454f-ba72-fdbd17c1ac0a&algo_exp_id=972b6314-ecbe-454f-ba72-fdbd17c1ac0a-16&pdp_ext_f=%7B%22sku_id%22%3A%2212000025643491627%22%7D&pdp_npi=2%40dis%21CAD%219.83%219.05%21%21%214.77%21%21%402101fd4b16674129711452727e8447%2112000025643491627%21sea&curPageLogUid=smF4Ng8fFxL6

 

[stok]

My 2 Cents worth of info...
the ups will function normal with a lifepo4 battery until the battery gets discharged to a point that the bms disconnects the load ( the ups cut off min voltage is almost always lower than the bms low voltage disconnect) and when this happens the ups does not sense a battery connected to it as the bms fets are open and and battery circuit is not complete. The ups senses for a voltage before it will start charging, and if it cannot pick anything up ( due to the low voltage disconnect from the bms) it will not power on again until you disconnect the battery from the ups and reconnect it to reset the LVD. this is all BMS dependent the vast majority of "cheap" bmse's works this way.

either install a DC rated circuit breaker between the ups and battery to act as a reset switch to disconnect and reconnect the battery from the ups in order to reset the LVD condition of the bms.

or

you can put a resistor in parallel over the bms p- and b- ports, the resistor basically "by passes" the bms (use a high value min 500 ohms and for safety's sake use a high wattage resistor in this case of a 24v ups with a 500 ohm resistor it will pass 1.25 watts of power the, the higher the resistor value the lower the wattage through the resistor.
it will let a very small amount of current(0.005 amps in case of 24v ) to flow regardless of the bms condition, but the small amount is not enough to start the ups, it is just enough for the UPS to sense that there is a battery connected and it can start to charge the battery.

I have done plenty of this type of ups conversions on all different brands of ups'es.

most cheap bms'es work this way except DALY

I always spec the bms according to the wattage of the unit, a 750va/500w ups can only draw 500 watts nothing more, the 750VA rating is like saying its 500 watt rms and 750 watt p.m.p.o in speaker terms....

 

[OffGridInTheCity]

On another note have you tried any of these cases that you can purchase from Aliexpress and put in 3S7P battery packs (12.6V)? I am trying them but not sure if they will provide enough runtime (still testing this).

When you say 3s7p - if you're talking about lithium-ion (18650 cells) then this won't work well. Lithium-ion 3s is too low a voltage range for a 12v substitute and 4s is too high. 7s for 24v or 14s for 48v substitutes work fine.
For LifePo4 - you need 4s for a 12v substitute.

As for running a battery down and BMS shuts-off - then perhaps you can recover the situation by using an external charger to bring it back up. In my APC UPS cases, the goal is to smooth off-grid / grid switch-overs or short grid dips. The idea is to cover blips and have a few mins to gracefully shut computers / sensitive equipment down.