Lead acid charging in series

[CntrlAltDel]

Hi there,

I have very basic knowledge about lead acid batteries and I've been researching to find out how exactly one should be charging them. I've recently bought 2x 12v 9ah SLA batteries to replace my UPS batteries with. I use the UPS to handle switch from grid power to battery power from my EcoFlow Delta (since the Delta doesn't switch fast enough to keep my PC on). They worked fine for 3-4 months but now when the power draw is about 600w everything shuts down and it cant handle the switch.

I know it's not the UPS itself because I've tried 3 different brands of UPS's and they all behave the same. The moment the grid power drops and the UPS has to kick in for a second or two to get power from the lithium battery bank it either powers off my monitors or my whole PC. And all three of the UPS's are rated to handle up to 1200w power draw.

I've gotten an SLA charger that charges the batteries to 13.8v each but now I've seen people say it's better to charge them in series at 24v since they are used in series in the UPS.

My questions are:
- Should I charge them at 24v or 27.6v in series at 1.2a?
- Should I charge them one by one at 13.8v 1.2a?
- How frequently should I remove them from the UPS to give them a proper charge?
- If I'm completely wrong about the above, how should I be doing it?

 

[Checkthisout]

Hi there,

I have very basic knowledge about lead acid batteries and I've been researching to find out how exactly one should be charging them. I've recently bought 2x 12v 9ah SLA batteries to replace my UPS batteries with. I use the UPS to handle switch from grid power to battery power from my EcoFlow Delta (since the Delta doesn't switch fast enough to keep my PC on). They worked fine for 3-4 months but now when the power draw is about 600w everything shuts down and it cant handle the switch.

I know it's not the UPS itself because I've tried 3 different brands of UPS's and they all behave the same. The moment the grid power drops and the UPS has to kick in for a second or two to get power from the lithium battery bank it either powers off my monitors or my whole PC. And all three of the UPS's are rated to handle up to 1200w power draw.

I've gotten an SLA charger that charges the batteries to 13.8v each but now I've seen people say it's better to charge them in series at 24v since they are used in series in the UPS.

My questions are:
- Should I charge them at 24v or 27.6v in series at 1.2a?
- Should I charge them one by one at 13.8v 1.2a?
- How frequently should I remove them from the UPS to give them a proper charge?
- If I'm completely wrong about the above, how should I be doing it?

I would do them each individually.

Your charging voltage needs to be much higher than 13.8 or or 24 volts in either case.

You need at least 14.4 individual or 28.8 series.

To keep them topped off when not in use I would hit them with 14.4 or 28.8 every few days for 15 minutes or so.

 

[CntrlAltDel]

I would do them each individually.

Your charging voltage needs to be much higher than 13.8 or or 24 volts in either case.

You need at least 14.4 individual or 28.8 series.

To keep them topped off when not in use I would hit them with 14.4 or 28.8 every few days for 15 minutes or so.

Thanks for this response. I've gone with charging them in series at 28.8v using a DC PSU with a buck booster to clamp voltage and current. Seems it's only charging at 0.2a and when I try 14.4v in single it only charges at max 0.1a. This is very strange.

 

[SamDeleted]

Thanks for this response. I've gone with charging them in series at 28.8v using a DC PSU with a buck booster to clamp voltage and current. Seems it's only charging at 0.2a and when I try 14.4v in single it only charges at max 0.1a. This is very strange.

Have you tried a dedicated sla/AGM/gel charger ?

 

[CntrlAltDel]

Have you tried a dedicated sla/AGM/gel charger ?

Yes, I have one that does 13.8v charges. Based on the response from @Checkthisout I think that may have been the reason why my batteries aren't working optimally.

 

[SamDeleted]

Yes, I have one that does 13.8v charges. Based on the response from @Checkthisout I think that may have been the reason why my batteries aren't working optimally.

Yes could be , I'm not quite sure what's going on with your batteries...

 

[SamDeleted]

It could be they're not taking current because they are already fully charged ?

Try cycling them a few times , full to empty (10.5v low cut off)

 

[CntrlAltDel]

It could be they're not taking current because they are already fully charged ?

Try cycling them a few times , full to empty (10.5v low cut off)

Good idea. I got a couple of 220mm 12v fans I could attach to them both and let them run for a few hours. Thanks for your response.

 

[SamDeleted]

Good idea. I got a couple of 220mm 12v fans I could attach to them both and let them run for a few hours. Thanks for your response.

Yes sounds good, It's a nice opportunity to test their Ah capacity too

 

[CntrlAltDel]

@Checkthisout @SamG340 Turns out the buck booster is actually dead that's why it couldn't charge about 0.2a. I've managed discharge one of the batteries at 60w using the EcoFlow's solar charge input and now I'm currently draining it to 10.5v using the 220m fans.
I've ordered 2 more buck boosters which should be with me Tuesday then I'll charge both the batteries at 14.4v @ 1.2a each.

 

[SamDeleted]

@Checkthisout @SamG340 Turns out the buck booster is actually dead that's why it couldn't charge about 0.2a. I've managed discharge one of the batteries at 60w using the EcoFlow's solar charge input and now I'm currently draining it to 10.5v using the 220m fans.
I've ordered 2 more buck boosters which should be with me Tuesday then I'll charge both the batteries at 14.4v @ 1.2a each.

24hrs on charge if you can

 

[CntrlAltDel]

24hrs on charge if you can

Could you confirm how do I check for the current battery level? Do I test voltage with a load (i.e fans) on the battery and take that reading or without the load? Without a load it seems to be roughly 1-1.5v higher.

 

[SamDeleted]

Could you confirm how do I check for the current battery level? Do I test voltage with a load (i.e fans) on the battery and take that reading or without the load? Without a load it seems to be roughly 1-1.5v higher.

Voltage is a poor indicator of battery soc (state of charge) because of the swing when charging & discharging . Really you need a shunt to accurately measure


This chart is for resting voltage (upto 24hrs no charge/discharge)

 

[CntrlAltDel]

Voltage is a poor indicator of battery soc (state of charge) because of the swing when charging & discharging . Really you need a shunt to accurately measure


This chart is for resting voltage (upto 24hrs no charge/discharge)
View attachment 166954

One of the batteries was just on 10.5v with no load so I thought I was done discharging it. I checked 30 minutes later and it's now 11.52v this is very confusing.

Also can I just leave 1x12v 0.4a fan connected to each battery so it can drain overnight? Will it stop drawing power at 10.5v / 0% capacity or will it discharge further essentially damaging the battery?

 

[SamDeleted]

One of the batteries was just on 10.5v with no load so I thought I was done discharging it. I checked 30 minutes later and it's now 11.52v this is very confusing.

Also can I just leave 1x12v 0.4a fan connected to each battery so it can drain overnight? Will it stop drawing power at 10.5v / 0% capacity or will it discharge further essentially damaging the battery?

It will damage the battery, you need an extra circuit to 'low volt cutout' , some equipment has that built in, your fans almost certainly DONT




Yes it take a long time for the voltage to recover after a load has been on, hours possibly even a day .

Hitting 10.5v with a load on and then raising to 11.5v once load is off sounds very normal to me , that is an flat battery


11.5v at resting would be 10% SOC

 

[Vigo]

I have a question.. why aren't the UPS's themselves charging your batteries? Not that you couldn't improve upon the onboard charging regime, but i don't understand why they are being charged 'manually' to begin with.

 

[CntrlAltDel]

I have a question.. why aren't the UPS's themselves charging your batteries? Not that you couldn't improve upon the onboard charging regime, but i don't understand why they are being charged 'manually' to begin with.

I think it's the same with all these types of UPS's. They work well for 3-4 months and then either you replace the batteries or charge them properly. I thought this was the general consensus with these things. Then again I'm not too knowledgeable about this topic.

 

[Vigo]

So i think a lot of cheaper UPS’s give the battery just a simple ‘float only’ type of charge which isn’t ideal for long term battery health but doesn't 'require' you to implement an external charging scheme would be if you are spending so much time running off battery that you're 'outrunning' the ability of the onboard charging to keep up during the time window it has to recharge.

It's just something that caught my curiosity.. if you care to share any model #s i could maybe say more, but the whole subject is just a tangent to the question you actually asked so feel free to ignore me!

 

[CntrlAltDel]

So i think a lot of cheaper UPS’s give the battery just a simple ‘float only’ type of charge which isn’t ideal for long term battery health but doesn't 'require' you to implement an external charging scheme would be if you are spending so much time running off battery that you're 'outrunning' the ability of the onboard charging to keep up during the time window it has to recharge.

It's just something that caught my curiosity.. if you care to share any model #s i could maybe say more, but the whole subject is just a tangent to the question you actually asked so feel free to ignore me!

Sure, here are the UPS's that I have and all have the exact same issue.
Eaton 5E850IUSB
Mecer ME-2000-VU
LinkQNet UPS-INT-2000VA-LQ-K

The replacement batteries I have for these are:
Forbatt AGM 12V VRLA 9AH FB12-9

@SamG340 Seems like I cant really get the batteries to charge at above 27.74v to 28.8v without increasing the amperage to about 3.30a. They're currentlu at 27.74v charging at 2.10a. When I disconnect them from charge they immediately start to drop in voltage so not sure if this is normal behaviour. But I'll be charging them for another 12 hours to make sure they reach the 24 hour mark like you suggested.

 

[SamDeleted]

Sure, here are the UPS's that I have and all have the exact same issue.
Eaton 5E850IUSB
Mecer ME-2000-VU
LinkQNet UPS-INT-2000VA-LQ-K

The replacement batteries I have for these are:
Forbatt AGM 12V VRLA 9AH FB12-9

@SamG340 Seems like I cant really get the batteries to charge at above 27.74v to 28.8v without increasing the amperage to about 3.30a. They're currentlu at 27.74v charging at 2.10a. When I disconnect them from charge they immediately start to drop in voltage so not sure if this is normal behaviour. But I'll be charging them for another 12 hours to make sure they reach the 24 hour mark like you suggested.

Charging manually with a psu is complicated process, it's doable , but difficult

So looking at it logically, if there's anything wrong with your batteries, the first thing to eliminate is the possibility that you're doing something wrong with that PSU.

Get yourself an AGM/gel smart charger and see if that makes a difference , CTEK are great