Using lithium batteries in applications designed for lead-acid

[Crossy]

In this instance small UPSs, but the principles apply to all kinds of application.

I put this in the Alternative Energy Forum as that's where the battery experts seem to be most active ???? 

 

Background

We have a number of UPSs of various sizes scattered around our home. The important one keeps my desktop, NAS and network kit alive others operate the grandkids PCs and various TV sets (can't interupt Madam's Thai soaps).

Of course this means that there's a steady stream of replacement batteries required. Whilst it's hardly a strain on the budget (and the recycling man is very happy to take away dead batteries) it is another thing to worry about.

Last straw was when the power went out during an "important" show, the UPS didn't come on and Madam missed about a minute whilst the genset started.

So, new battery required. This time I decided to go with one of the plethora of drop-in lithium replacements. Rather than go ready-made I decided to DIY so I know what's in the box.

 

The solution

A bit of shopping got me:-

4 x 6,000mAh TipSun 32700 LiFePO4 cells (actually the ones from my aborted solar lighting experiment).

1 x 4S, 50A LiFePO4 BMS

1 x Battery box which is the same size as the 12V 7.5Ah lead-acid batteries I use for the UPSs (includes 32700 battery holder)

1 x Bag of copper battery interconnect strips

 

Total cost = 805 Baht from Lazada seller The Power Idea. For comparison the 7.5Ah lead-acid battery is 625 Baht from HomePro (cheaper on Lazada of course).

 

Half an hour later we have this:-

 

The guts.

 

 

In the box.

 

 

Testing

I charged the beast using my LiFe charger to ensure it was full-full and nailed it on to one of the baby UPSs that supplies a small PC, my DVR and a router (about 100W or so).

 

Pulled the power to the UPS and started the timer, 45 minutes later the BMS shut off on over-discharge. Most satisfactory.

 

Problem No.1 - Plugged the UPS back in and nothing happened ???? 

BMS stayed off and the UPS refused to do anything, probably due to it thinking its battery was dead-dead.

Disconnect the UPS from the battery and things came back to life, reconnect and the battery started charging OK.

 

Left things alone overnight to ensure the battery was as fully charged as the UPS internal charger could get it.

This time it ran the load for 35 minutes.

 

So the internal charger:-

• Won't start with no battery in place.
• Doesn't fully charge the higher voltage of the LiFePO4 pack.

The above aren't an issue if you never run the battery flat or need the full available runtime (which we generally don't, but there's a solution to both).   

 

More coming soon.

 

 

 

[cleopatra2]

2 hours ago, Crossy said:

In this instance small UPSs, but the principles apply to all kinds of application.

I put this in the Alternative Energy Forum as that's where the battery experts seem to be most active ???? 

 

Background

We have a number of UPSs of various sizes scattered around our home. The important one keeps my desktop, NAS and network kit alive others operate the grandkids PCs and various TV sets (can't interupt Madam's Thai soaps).

Of course this means that there's a steady stream of replacement batteries required. Whilst it's hardly a strain on the budget (and the recycling man is very happy to take away dead batteries) it is another thing to worry about.

Last straw was when the power went out during an "important" show, the UPS didn't come on and Madam missed about a minute whilst the genset started.

So, new battery required. This time I decided to go with one of the plethora of drop-in lithium replacements. Rather than go ready-made I decided to DIY so I know what's in the box.

 

The solution

A bit of shopping got me:-

4 x 6,000mAh TipSun 32700 LiFePO4 cells (actually the ones from my aborted solar lighting experiment).

1 x 4S, 50A LiFePO4 BMS

1 x Battery box which is the same size as the 12V 7.5Ah lead-acid batteries I use for the UPSs (includes 32700 battery holder)

1 x Bag of copper battery interconnect strips

 

Total cost = 805 Baht from Lazada seller The Power Idea. For comparison the 7.5Ah lead-acid battery is 625 Baht from HomePro (cheaper on Lazada of course).

 

Half an hour later we have this:-

 

The guts.

 

 

In the box.

 

 

Testing

I charged the beast using my LiFe charger to ensure it was full-full and nailed it on to one of the baby UPSs that supplies a small PC, my DVR and a router (about 100W or so).

 

Pulled the power to the UPS and started the timer, 45 minutes later the BMS shut off on over-discharge. Most satisfactory.

 

Problem No.1 - Plugged the UPS back in and nothing happened ???? 

BMS stayed off and the UPS refused to do anything, probably due to it thinking its battery was dead-dead.

Disconnect the UPS from the battery and things came back to life, reconnect and the battery started charging OK.

 

Left things alone overnight to ensure the battery was as fully charged as the UPS internal charger could get it.

This time it ran the load for 35 minutes.

 

So the internal charger:-

• Won't start with no battery in place.
• Doesn't fully charge the higher voltage of the LiFePO4 pack.

The above aren't an issue if you never run the battery flat or need the full available runtime (which we generally don't, but there's a solution to both).   

 

More coming soon.

 

 

 

I am surprised you chose to go this route.

As you probably aware the charging algorithm for lithium and lead acid are different.

In the case of using a charger designed for lead acids on lithium batteries the equalisation mode needs to be disabled.

The lead acid charger will not fully maintain the charge/store of lithium batteries . Thus the charger should be disconnected to prevent damage once the lithium's have become charged.

[Crossy]

@cleopatra2 you make excellent points, indeed you can't just bung in lithium as replacement for lead acid. The cells will pretty rapidly become non-cells ???? 

 

The LiFePO4 BMS takes care of any over-charge or over-discharge issues and also provides a (low current) cell balance function. Of course this only works correctly if the charger actually has sufficient float voltage to fully charge the lithium pack (to be addressed in a later post).

 

 

 

[cleopatra2]

1 hour ago, Crossy said:

@cleopatra2 you make excellent points, indeed you can't just bung in lithium as replacement for lead acid. The cells will pretty rapidly become non-cells ???? 

 

The LiFePO4 BMS takes care of any over-charge or over-discharge issues and also provides a (low current) cell balance function. Of course this only works correctly if the charger actually has sufficient float voltage to fully charge the lithium pack (to be addressed in a later post).

 

 

 

Whilst the BMS will go into open circuit mode when it detects a high voltage. It will not prevent overcharging by holding the constant voltage phase of the charging algorithm for longer than is required. This would likely lead to lithium plating.

[Crossy]

6 minutes ago, cleopatra2 said:

Whilst the BMS will go into open circuit mode when it detects a high voltage. It will not prevent overcharging by holding the constant voltage phase of the charging algorithm for longer than is required. This would likely lead to lithium plating.

 

Definitely more reading required on this subject.

[maxpower]

Add CC CV circuit to suit the 4s battery and all will be fine. If a balance circuit is present you can adjust the output to suit its requirement.

 

I guess you have lost the UPS low battery cut off and the BMS gets it first. On some UPS this can be overcome by tweaking the battery voltage sense circuit.

 

Unless this UPS gets called upon often, I would not bother too much over the BMS cutting off. The voltage might rise above recovery once the load has gone away.

[Crossy]

I've got 3 (soon 4) small UPSs with these packs (one is using 5Ah McNair cells) all achieve about 13.6V (3.4V per cell) with the unmodified charger. I think I'm going to leave them alone at this stage, they will rarely be called upon to run for more than a few minutes so running to cut-off isn't a problem even if I have to re-fuel the genset.

 

The bigger 24V units are being more fun, more of these later.

 

The phenomenon of lithium-plating doesn't get mentioned much, all the papers I can find are talking about fast charging and/or low temperatures neither of which apply here. If anyone has more information particularly where CV float chargers are involved it would be interesting and useful. There must be literally millions of these "drop-in" lead-acid replacements in use with simple BMSs and trickle/float chargers, if it's really an issue why the lack of information.

 

[Crossy]

As promised, the 24V UPS update.

 

I made up two 8S packs, one with 8x5Ah McNair cells (for 24V @ 5Ah) and one with 16x5Ah McNair cells (for 24V @ 10Ah). Both have the same 8S 60A BMS and XT90 connector. Excuse the crumby soldering although it's much better than some I see as "good" on Youtube videos.

 

 

 

I also got some CC/CV charger boards @ 75 Baht

https://www.lazada.co.th/products/i1591488632-s4324320455.html

A 24V 3A switching power supply (adjusted to 30V) was in stock.

 

 

 

I have two 24V UPSs, an APC 800W/1600VA unit which powers my desktop, NAS and network kit and a Leonics "Green" 600W/1000VA which keeps the larger of the kids machines and a 40" TV alive.

 

Surgery on the APC

The 24V PSU and charger board fit where the existing lead-acid batteries lived, there's actually room for the 8 cell pack in there too, but I decided that it would be happier out in the fresh air so the battery leads exit via a new hole in the back of the case.

 

There's also a 20A (from stock, a 5A unit would be fine) diode between the power supply and the charger board to prevent the batteries back-feeding the PSU and possibly causing damage.

 

 

 

Charger board set to 28.8V (8*3.6V) and 3A (max recommended). The power supply is wired direct to the mains inlet socket.

 

Testing

Both packs were top-balanced (all cells fully charged) using my LiFePO4 charger.

I have a 500W halogen floodlight as a dummy load (verified with my Kill-A-Watt as 490W, close enough).

 

500W at 24V is about 21A so the 8 cell pack would be discharging at >4C, in reality the current drawn was about 24A (nearly 5C) so evidently the inverter isn't awfully efficient (evidenced by how "warm" it gets).

 

Anyway, all hooked up, mains off, timer on. After 9 minutes the BMS turned off, which was probably best as the cells were HOT!!. I certainly would not want to be running them regularly at that discharge rate / time although in my application with a generator backup they would never actually have such a long run-time.

 

After things had cooled off (including myself with a cold Chang) the 16 cell pack was tested in the same way. At a much kinder 2.5C discharge the cells were warm to touch but not crazy hot and it lasted 25 minutes before shutting off.

 

The load on the APC in use is about 300-350W (depends how hard the PC and NAS are working) so it should be good for 30 minutes, plenty of time for me to re-fuel the genset or find out why it didn't start.

 

Annoyances

The APC is too smart for it's own good. 

 

It has a battery-life monitor which flashes a "replace battery" light although the unit continues to function just fine (tape over the lamp). Unfortunately it also beeps regularly to encourage battery replacement, I was unable to turn this function off so the beeper was removed (it was the last of my UPSs to be de-beeped).

 

Also the self-test function from the PC software no longer works and of course the run-time monitor is way off.

 

All good fun ???? 

 

 

 

 

 

[maxpower]

2 hours ago, Crossy said:

Excuse the crumby soldering