Is it safe to add another charger to lifepo4 battery connected to a UPS?

[narmenia]

The ups is rated at 650VA. This is a cheap 27$ ups.

Super Fast Charging Line Interactive Series | PROLiNK

PRO701SFC

prolink2u.com

Stock battery is a single tiny 7AH Seal Lead Acid.

I jerry rigged a 90AH lifepo4 battery and installed a fan to prevent the UPS from overheating during long charge and discharge.
BMS is 10a discarge and 5a charge.

The setup is OK for my usage but charging is really slow.
It has been 3days now but the battery is still at 13.26v.
Im afraid if i get succeeding 8hrs blackouts the ups would not be able to charge quickly and i lose power.

I am thinking of buying a 5a 14.6volt charger to supplement the charging of the ups.
Would it work? UPS charging the battery plus a 2nd charger?

 

[narmenia]

up

 

[Maast]

You can parallel chargers as much as you want - as long as you dont exceed the charging C rating of the batteries, and you make sure you dont overvolt the UPS.
I've done the exact same thing (modify an UPS w/ external batteries) and as you found out the charging is painfully slow so I added a external charger.

 

[narmenia]

You can parallel chargers as much as you want - as long as you dont exceed the charging C rating of the batteries, and you make sure you dont overvolt the UPS.
I've done the exact same thing (modify an UPS w/ external batteries) and as you found out the charging is painfully slow so I added a external charger.

make sure you dont overvolt the UPS.

can you elaborate on this?
does this mean that my charger must be the same voltage as what my ups charges?

My ups charges at 13.6 volts. As the battery would just sit at this voltage and would not go up anymore.
I am not sure the charging amperage of the ups but i imagine it is around ~1.5A (or less).
As the UPS has a 7AH battery and the advertisement says:

SUPER FAST CHARGING​

Battery can be recharged to 90% of its capacity within 2-4 hours.

My Lifepo4 battery charger has arrived.
The description says 14.6 volts @ 10 amps
But i tested the voltage with my multimeter its @ 14.7 volts.

I am planning to use a charging controller (XH-M602)
As i am not sure how the charger would charge the battery (im afraid it would just sit at 14.7 volts forever)

 

[Positronium]

I went through a similar scenario. I wanted to add a feature to one of my projects that would allow to charge 4s LiFePO4 battery using 100W USB power delivery charger (capable of 5A @ 20V). CC-CV buck converters and USB-C trigger boards and can be bought on Amazon, I have attached the ones I picked. The thing is that buck converters will keep the constant voltage they are set to, which one is tempted to set it at 14.4V, the absorption voltage of a 4s LiFePO4 battery; That is problematic as these buck converters do not switch to float charge (typically 13.4V - 13.6V) like most lead acid chargers or smart LiFePO4 chargers that switch to float voltage while the less expensive ones will just stop charging.

Maintaining LiFePO4 and SLA battery at 14.4V will damage both SLA and LiFePO4 batteries. In my setup I set the constant current of 5A and constant voltage of 13.55V on the buck converter. Now I can safely charge my 4s LiFePO4 at 60W from a 100W USB-C PD charger.

Review your LiFePO4 charger manual or specifications as you may not need the charge control board board look for if it is 4 step charger (recovery, bulk, absorption, and float) review the voltage value for float. If you cannot verify from manual or specifications of charger then consider a the charge control board. Set your charge control board's max voltage to 13.55V and you will be fine as both LiFePO4 and SLA are fine with that float voltage.

 

[Maast]

can you elaborate on this?
does this mean that my charger must be the same voltage as what my ups charges?

The UPSs I've messed with (APC SURTA and SUAs) have all charged at a low constant current. By overvolting the UPS I meant the particular UPS may not like higher voltages than what it's designed to output. Depending on your model it may throw a overvoltage error and shut down, or just ignore it. No what to tell ahead of time.

'Smart' chargers increase the voltage to keep the battery-vs-charger voltage differential to pump the maximum amount of amperage it can into the battery. That means that near the top of the charge slope it may be putting out 15 or 16 volts before it reduces the voltage to keep from overcharging the battery. Check out the link in my signature about charging for more details.