Thread Starter

anditechnovire

Joined Dec 24, 2019
105
Please I have a bunch of lithium batteries connected in parallel, each rated 3.2v and 3Ah.(about 4 of them. I have a laptop charger with an output of 19v at 2.1A. I know I can't connect it directly to the batteries, so what type of circuit can I use to charge the batteries via the laptop charger. Is it a buck converter or what else is best.
 

Alec_t

Joined Sep 17, 2013
12,744
To prevent fire or explosion, lithium-based batteries need special voltage/current /time charging arrangements. There are dedicated bms (battery management system) ICs which can control that.
It will probably be cheaper to buy an off-the-shelf proper charger rather than try to build your own.
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
105
To prevent fire or explosion, lithium-based batteries need special voltage/current /time charging arrangements. There are dedicated bms (battery management system) ICs which can control that.
It will probably be cheaper to buy an off-the-shelf proper charger rather than try to build your own.
Yeah, I was asking what type of circuit I can use, not what to build. I've not planned to build any circuit on my own. So I've read about BMS boards but was skeptical about it cause it looked very small to me to manage such battery capacity.
So let go with BMS board. Can I use it along with the laptop charger (19v 2.1A). And please what specifications will I need.
Thank you
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
105
Those would be LiFePo4 cells. Make sure to get a charger for that type rather than the standard LiI Ion type.

Bob
Okay thank you. Is there a different charger for LiFePo4 cells. Thought I just needed a voltage source, then an appropriate circuit acting as a charge controller.
But can't I use the laptop charger along with a BMS board or any other circuit.
 

Alec_t

Joined Sep 17, 2013
12,744
Thought I just needed a voltage source, then an appropriate circuit acting as a charge controller.
19V from a laptop supply would first have to be stepped down to a voltage closer to 3.2V. That voltage, and the charging current, would both have to be properly controlled as a function of time.
A quick bit of googling found that most commercial chargers are intended for battery voltages higher than 3.2V, e.g.12V or 24V. The write-ups for 3.2V charging modules I saw advertised did not specify the module input voltage or any current limit lower than the 2A which your laptop supply could handle.
With your battery capacity of 4 x 9.5Ah = 38Ah, even if your laptop supply could be used somehow it would take over 19 hours for a full charge.
 

Audioguru again

Joined Oct 21, 2019
4,537
DO NOT simply guess what is the charging voltage. Instead read the datasheet for the exact battery you have.
I have never seen a charger or BMS made for the newer LiFeP04 batteries.

Your laptop charger voltage is much too high and its current is much too low. Don't use it.
 

bassbindevil

Joined Jan 23, 2014
373
Those little "BMS" boards are just for protection from overcharging, over-discharging, and short circuits. At best they also provide cell balancing, but that's not always included (and irrelevant since your cells are in parallel). There's a pretty good chance that a buck converter would work if it is adjusted to the correct final voltage, and has adjustable current limiting. The current limiting is mainly to prevent overloading the power brick, since your cells could probably be safely charged at a much higher rate.
 

Audioguru again

Joined Oct 21, 2019
4,537
A charger properly detects a full charge then disconnects to avoid overcharging.
A Buck converter continues overcharging until the battery is ruined.
When the battery voltage reaches its recommended highest voltage then it is not yet fully charged. The dropping of charging current is detected as a full charge. But a buck converter dont doo dat.
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
105
A charger properly detects a full charge then disconnects to avoid overcharging.
A Buck converter continues overcharging until the battery is ruined.
When the battery voltage reaches its recommended highest voltage then it is not yet fully charged. The dropping of charging current is detected as a full charge. But a buck converter dont doo dat.
So what's your own suggestion please.
Should I discard the batteries all because there's no way to charge it 100 efficiently.
When you kick against all recommendations, please endeavour to bring yours cause a solution is actually what I'm looking for.
 

Audioguru again

Joined Oct 21, 2019
4,537
You need a proper charger made for your odd new kind of batteries. Keep looking for one.
A proper charger detects a full charge then shuts off.

A lithium battery is about 70% fully charged when its voltage reaches the maximum allowed voltage. Then you need a circuit to detect that and shut off the charging.
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
105
But
You need a proper charger made for your odd new kind of batteries. Keep looking for one.
A proper charger detects a full charge then shuts off.

A lithium battery is about 70% fully charged when its voltage reaches the maximum allowed voltage. Then you need a circuit to detect that and shut off the charging.
I can manage without the overcharged protection for now right. They're just some old batteries and I really need to use it urgently.
The battery pack in question came with it own BMS, but the same type is not in the retail market.
I can always test it manually every time I charge it. But can't I build a simple circuit for that?
 

Audioguru again

Joined Oct 21, 2019
4,537
There are many videos of little Lithium batteries charged wrong and exploding but your batteries are much larger (more powerful) so I would say do not charge them with a simple DIY circuit.
 

Irving

Joined Jan 30, 2016
2,546
So much misinformation...

If these ARE LiFePO4 there is virtually no risk of fire or explosion. There is a risk of contamination should they leak, but in 10y of mistreating LiFePO4 cells I've yet to see a leak except when I deliberately drive a nail through one by way of demonstration...

To charge them you need a constant current charge, typically between 0.5 - 5C (ie for a 10Ah cell, between 5A and 50A) until the voltage hits 3.65v +/-5mV (for max charge & longevity), then constant voltage until the charge current drops to typically 0.02C (ie 200mA for a 10Ah cell). Looked after, a LiFePO4 cell will last 10 - 15y with >80% original capacity, much better than other Li-ion technologies.

For a 1 - 4 cells in series up to 1 or 2A charge rate there are several off the shelf chargers that work OK, either dedicated LiFePO4 or dual technology with a LiFePO4 switch. Unless it has a way of specifically selecting LiFePO4 cells, ignore chargers that say ok for Li-ion/LiFePO4 3.7/3.6v - these will overcharge a LiFePO4 and undercharge an Li-Po cell.

It is hard to find a 1 - 4S charging board/BMS for LiFePO4. I've rolled my own before. If you can find one using the TP5000 chip this is good for 1S charger (reviewed).

For 1 - 8 cells in series, up to 40A charge rate there's only 1 charger I rate - Revolectrix' PL8 (I own 4 of them now), though other brands are available.

For individual cell charging its easy to do with a good bench supply and a multimeter. Like this:
  1. Turn on bench supply output and set supply output volts to 3.62 - 3.65v (measured, don't rely on display);
  2. Short output terminals and set required charge current on supply. Use appropriate wire for the charge current, ie 8 or 10AWG for 30-50A, 12AWG for 10-30A, etc;
  3. Turn off output, attach partly discharged cell (but not if cell volts much below 2.5v, these need to be pre-soaked at 50-100mA until they reach 3v first);
  4. Turn on output, supply should go into current limit and volts drop to cell volts initially, then start climbing.
  5. When cell volts reaches 3.65v & supply comes out of current limit, monitor charge current till it drops to 0.02C, or 10 - 16h elapsed, whichever is sooner, then stop charging.

If you have several identical cells, you can group them into sets of 3, 4 or 5 (= N) of similar voltage, ie within a 50 - 75mV range, and charge in parallel using (N)x the currents above. Make sure the connections between them are sound and tight over the whole charge cycle and be careful to connect them all the same way round, ie + to +, - to -.
 
Last edited:

Thread Starter

anditechnovire

Joined Dec 24, 2019
105
So much misinformation...

If these ARE LiFePO4 there is virtually no risk of fire or explosion. There is a risk of contamination should they leak, but in 10y of mistreating LiFePO4 cells I've yet to see a leak except when I deliberately drive a nail through one by way of demonstration...

To charge them you need a constant current charge, typically between 0.5 - 5C (ie for a 10Ah cell, between 5A and 50A) until the voltage hits 3.65v +/-5mV (for max charge & longevity), then constant voltage until the charge current drops to typically 0.02C (ie 200mA for a 10Ah cell). Looked after, a LiFePO4 cell will last 10 - 15y with >80% original capacity, much better than other Li-ion technologies.

For a 1 - 4 cells in series up to 1 or 2A charge rate there are several off the shelf chargers that work OK, either dedicated LiFePO4 or dual technology with a LiFePO4 switch. Unless it has a way of specifically selecting LiFePO4 cells, ignore chargers that say ok for Li-ion/LiFePO4 3.7/3.6v - these will overcharge a LiFePO4 and undercharge an Li-Po cell.

It is hard to find a 1 - 4S charging board/BMS for LiFePO4. I've rolled my own before. If you can find one using the TP5000 chip this is good for 1S charger (reviewed).

For 1 - 8 cells in series, up to 40A charge rate there's only 1 charger I rate - Revolectrix' PL8 (I own 4 of them now), though other brands are available.

For individual cell charging its easy to do with a good bench supply and a multimeter. Like this:
  1. Turn on bench supply output and set supply output volts to 3.62 - 3.65v (measured, don't rely on display);
  2. Short output terminals and set required charge current on supply. Use appropriate wire for the charge current, ie 8 or 10AWG for 30-50A, 12AWG for 10-30A, etc;
  3. Turn off output, attach partly discharged cell (but not if cell volts much below 2.5v, these need to be pre-soaked at 50-100mA until they reach 3v first);
  4. Turn on output, supply should go into current limit and volts drop to cell volts initially, then start climbing.
  5. When cell volts reaches 3.65v & supply comes out of current limit, monitor charge current till it drops to 0.02C, or 10 - 16h elapsed, whichever is sooner, then stop charging.

If you have several identical cells, you can group them into sets of 3, 4 or 5 (= N) of similar voltage, ie within a 50 - 75mV range, and charge in parallel using (N)x the currents above. Make sure the connections between them are sound and tight over the whole charge cycle and be careful to connect them all the same way round, ie + to +, - to -.
Oh, my former boss is here.
seems you'll help me continue from where we stopped
but what if I can get something like this.
1636912993817.png
 

Thread Starter

anditechnovire

Joined Dec 24, 2019
105
I'll just use a buck converter with current limiting feature and then step down the 19v from the laptop charger to 3.6v to charge the cells. maybe individually. They're not pretty new cells (a bit old and weak) so they'll not take much time to charge up. And for the overcharge protection, I don't really give a f**k about it. I'll test the battery voltage manually with a meter till when I read about 3.6v then I'll disconnect it.
 

Irving

Joined Jan 30, 2016
2,546
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