Battery charger//discharger for lithium ion battery

Thread Starter

Yin Min

Joined Jun 24, 2019
27
Hi,
I have 4.2V, 6.8A li-ion battery pack (series+parallel). I want to charge the battery pack while connecting with load (while discharging). Can the battery charger be used for it? I am using ISDT T8 charger.
Any suggestion, please.

Thanks,
Yin Min
 

Alec_t

Joined Sep 17, 2013
12,153
The battery pack can't charge and discharge at the same time. While it is charging, if you need to power the load then the power for that would have to come from the charger or some other source.
 

Thread Starter

Yin Min

Joined Jun 24, 2019
27
The battery pack can't charge and discharge at the same time. While it is charging, if you need to power the load then the power for that would have to come from the charger or some other source.
Ok. I see. You meant to say if the battery pack is being charged with charger and connecting with load at the same time, the power from charger will go directly to load. Is that right? Then, can the charger be used like this? can have any problem in charger side?
Thank you, Alec.
 

gerty

Joined Aug 30, 2007
1,301
Ok. I see. You meant to say if the battery pack is being charged with charger and connecting with load at the same time, the power from charger will go directly to load. Is that right? Then, can the charger be used like this? can have any problem in charger side?
Thank you, Alec.

If the charger has sufficient current to power the load, yes that might happen. There is also the question do you want to use the charger as a power supply. Some charger won't turn on unless a battery is connected, and that's to prevent someone from connecting it backwards..Do you need the battery ?
 

Thread Starter

Yin Min

Joined Jun 24, 2019
27
If the charger has sufficient current to power the load, yes that might happen. There is also the question do you want to use the charger as a power supply. Some charger won't turn on unless a battery is connected, and that's to prevent someone from connecting it backwards..Do you need the battery ?
Actually, I want to charge the battery pack while the load is drawing current from the battery pack because I want to run the load for certain time. If I don't charge the battery at the same time, don't have enough power to load. So, I do need the battery. But the thing is my battery charger. I am not sure it can be used like this or not. First time I am using. Do you know the other charger can work with this kind of charging?
Thank you.
 

Irving

Joined Jan 30, 2016
1,863
You must use a charger that powers the load independently of charging the battery, ie it disconnects the battery from the load. Otherwise the charger cannot tell when the the battery has reached the voltage level to switch from CC to CV mode, leaving the battery permanently undercharged.

Most commercial chargers and eBay '1 chip, 1S' mini charging boards cannot do this. There are chips that do so, eg Analog's LT1769 which balances system load and charging current to keep within DC source current limits. Others actually provide a switching regulator for the system and a separate one for the charger element, with a MOSFET to switch battery into the supply position.
 

Thread Starter

Yin Min

Joined Jun 24, 2019
27
You must use a charger that powers the load independently of charging the battery, ie it disconnects the battery from the load. Otherwise the charger cannot tell when the the battery has reached the voltage level to switch from CC to CV mode, leaving the battery permanently undercharged.

Most commercial chargers and eBay '1 chip, 1S' mini charging boards cannot do this. There are chips that do so, eg Analog's LT1769 which balances system load and charging current to keep within DC source current limits. Others actually provide a switching regulator for the system and a separate one for the charger element, with a MOSFET to switch battery into the supply position.
Ok, I see. But I need to connect the battery with the load. My battery charger is balancing charger. ISDT T8. If I disconnect the balancing port, will it be ok? Is there any easy and safe way to check before I connect with my actual load?
 

Irving

Joined Jan 30, 2016
1,863
You only have single cell so no balancing needed. But your charger cannot charge battery and drive load as well. If its a decent charger, it will recognise the charging regime isn't progressing properly, e.g. voltage rising in CC mode, and current demand is constant, and will shut down with an error condition.

You need a proper power adapter that can provide both system load current and charging current and a charging IC that manages the link between the two regimes. Or an unregulated supply and a charging chip that provides a regulated supply to your load and manages the charging regime.

How big is your battery? That's a massive charger.

edit: found it, 6.8Ah... so max 3A charge. How big is your load?
 

Thread Starter

Yin Min

Joined Jun 24, 2019
27
You only have single cell so no balancing needed. But your charger cannot charge battery and drive load as well. If its a decent charger, it will recognise the charging regime isn't progressing properly, e.g. voltage rising in CC mode, and current demand is constant, and will shut down with an error condition.

You need a proper power adapter that can provide both system load current and charging current and a charging IC that manages the link between the two regimes. Or an unregulated supply and a charging chip that provides a regulated supply to your load and manages the charging regime.

How big is your battery? That's a massive charger.

edit: found it, 6.8Ah... so max 3A charge. How big is your load?
Ok. I see. Thanks for your suggestion and help. My battery is not single cell. It is series+parallel battery pack (about 40hA). Actually, I need balancing. My load is 12 to 13A and I want to run load for certain time. But battery power will not be enough. So, I need to charge the battery at the same time. Now, the thing is my battery charger. It much more depends on my battery charger. Am I correct? Thanks.
 

Irving

Joined Jan 30, 2016
1,863
In post #1 you said "I have 4.2V, 6.8A li-ion battery pack". The # of cells in series/parallel could be vital bit of info. So now we know it's 6P, but what is #S?

You need to explain in much more detail why you need to run on batteries AND mains at the same time and what the main supply is, and why you're not running on that all the time.

Irrespective of the size of the pack, you cannot charge with that charger and run a load at the same time. If you want to do that, ie an "online" style of battery backup or UPS arrangement you need a different charger and/or a different chemistry battery.

For safety a normal LiPo offline charger will shutdown or error after typically 5h if the battery it is charging doesn't appear to be responding to charge. Indeed it might not even start. To check there is a battery there of the right chemistry a good charger will send a small, often pulsed, charge to the battery and watch how the volts change. If they don't behave as expected it won't start charging.

You need a solution that powers your load from the main supply most of the time while the batteries are kept offline or charging, and only runs on batteries when the main power is NOT present.

If you have a 40Ah LiPo pack and a 13A load then you will need to fully recharge every 2.5h (to 80% DoD) - if you go below 80% your batteries will be toast in less than a year (~1000 recharge cycles). Even with that charger a full recharge and balance will take 4+ hours.

Your thinking is flawed. Look at it this way.. Just suppose you have some magic battery that can charge and discharge at the same time. Let's assume your pack Is 6S = 25v at 100% SoC (it will actually be 22.8v, but I'll use 25 to keep the math easy).

You have 25 X 40 = 1000Wh of energy stored.

You need 25 X 10 = 250Wh of energy. (to keep the math simple).

After 4h your batteries will be completely dead.

So let's add 500W of mains power.
in the first hour 250W is powering your load. 250W is going into charging your batteries. But NOT to 25% as you lose 10% approx in charging, so only 225 goes in charging them to 22.5%.

After 4 hours your batteries are 90% charged. After 5.5 hours they will be 100% charged (4h45m) & (maybe) balanced (+45min), but the load is being powered by the 500W supply, so the batteries can only be a backup in case the main supply fails for a short time (<3h in every ~8.5h).

Let's do the same with a 300W supply. 250W is going into the load, 50W into the batteries, which now take ~23h to recharge & balance.

If your supply is below 250W your batteries will never recharge. On a 200W supply your batteries will provide 50W to the load and will he depleted in 20h

But LiPo batteries hate being 100% charged all the time, their life will be quite short so you need a solution that keeps them at say 90%, but then your run time without mains power will be less (or you need bigger batteries).
 
Last edited:

Thread Starter

Yin Min

Joined Jun 24, 2019
27
In post #1 you said "I have 4.2V, 6.8A li-ion battery pack". The # of cells in series/parallel could be vital bit of info. So now we know it's 6P, but what is #S?

You need to explain in much more detail why you need to run on batteries AND mains at the same time and what the main supply is, and why you're not running on that all the time.

Irrespective of the size of the pack, you cannot charge with that charger and run a load at the same time. If you want to do that, ie an "online" style of battery backup or UPS arrangement you need a different charger and/or a different chemistry battery.

For safety a normal LiPo offline charger will shutdown or error after typically 5h if the battery it is charging doesn't appear to be responding to charge. Indeed it might not even start. To check there is a battery there of the right chemistry a good charger will send a small, often pulsed, charge to the battery and watch how the volts change. If they don't behave as expected it won't start charging.

You need a solution that powers your load from the main supply most of the time while the batteries are kept offline or charging, and only runs on batteries when the main power is NOT present.

If you have a 40Ah LiPo pack and a 13A load then you will need to fully recharge every 2.5h (to 80% DoD) - if you go below 80% your batteries will be toast in less than a year (~1000 recharge cycles). Even with that charger a full recharge and balance will take 4+ hours.

Your thinking is flawed. Look at it this way.. Just suppose you have some magic battery that can charge and discharge at the same time. Let's assume your pack Is 6S = 25v at 100% SoC (it will actually be 22.8v, but I'll use 25 to keep the math easy).

You have 25 X 40 = 1000Wh of energy stored.

You need 25 X 10 = 250Wh of energy. (to keep the math simple).

After 4h your batteries will be completely dead.

So let's add 500W of mains power.
in the first hour 250W is powering your load. 250W is going into charging your batteries. But NOT to 25% as you lose 10% approx in charging, so only 225 goes in charging them to 22.5%.

After 4 hours your batteries are 90% charged. After 5.5 hours they will be 100% charged (4h45m) & (maybe) balanced (+45min), but the load is being powered by the 500W supply, so the batteries can only be a backup in case the main supply fails for a short time (<3h in every ~8.5h).

Let's do the same with a 300W supply. 250W is going into the load, 50W into the batteries, which now take ~23h to recharge & balance.

If your supply is below 250W your batteries will never recharge. On a 200W supply your batteries will provide 50W to the load and will he depleted in 20h

But LiPo batteries hate being 100% charged all the time, their life will be quite short so you need a solution that keeps them at say 90%, but then your run time without mains power will be less (or you need bigger batteries).
Hi Irving,

Thank you so much for your detailed explanation and suggestion. It makes me more clear.
My battery pack is 2S6P. Normally, I used power supply to load. Now, I got battery charger and wanted to try with battery pack. Because my load current is large so I need to charge to maintain the battery power. Yesterday, I have tried with my charger and battery pack. I can charge the battery with charger while load is drawing current from the battery. Now can solve my issue :>

One thing I am not clear is that now I have 40.8Ah 2S6P battery pack. but individual cell is 6.8Ah. If I charge the battery pack (2S6P), can I charge more than 6.8A? lets say 15A charge current for 2S6P battery pack.

Thanks again.
Yin Min
 

Irving

Joined Jan 30, 2016
1,863
If your charger does let you do it to start, I suggest to wait up to 16h on load and see if it's still doing it. If your charger let's you go to 24h then I'd be very concerned about it's safety as a charger as it has no timeout on charging. Good for you in this scenario but dangerous for charging LiPo batteries generally.

Your cells are 6.8Ah, their C rating, is but if you parallel 6 of them then they can deliver 40.8Ah. Similarly, on charge the maximum recommended charge rate is 0.5C or 3.4A per cell or 20.4A for the pack. You can charge faster but it reduces battery life.

The problem I see here is your charger should be delivering a fixed current in the first part of the charge cycle but if your load is taking that current then your batteries are not charging - or not charging at the rate you think they are.

You say it's working now with the charger? Let's see...

What part of the charge cycle does the charger think it's on?
What charge current is it set to for the CC part of the cycle?
What charge current is it showing?
What battery voltage is it set to for the CV part of the cycle?
What battery voltage is it showing?
 

Thread Starter

Yin Min

Joined Jun 24, 2019
27
If your charger does let you do it to start, I suggest to wait up to 16h on load and see if it's still doing it. If your charger let's you go to 24h then I'd be very concerned about it's safety as a charger as it has no timeout on charging. Good for you in this scenario but dangerous for charging LiPo batteries generally.

Your cells are 6.8Ah, their C rating, is but if you parallel 6 of them then they can deliver 40.8Ah. Similarly, on charge the maximum recommended charge rate is 0.5C or 3.4A per cell or 20.4A for the pack. You can charge faster but it reduces battery life.

The problem I see here is your charger should be delivering a fixed current in the first part of the charge cycle but if your load is taking that current then your batteries are not charging - or not charging at the rate you think they are.

You say it's working now with the charger? Let's see...

What part of the charge cycle does the charger think it's on?
What charge current is it set to for the CC part of the cycle?
What charge current is it showing?
What battery voltage is it set to for the CV part of the cycle?
What battery voltage is it showing?
Thanks for your suggestion. My load testing time is maximum 2 hours only. So, I want to maintain the battery power within 2 hours. If the battery is fully charged, the charger won't charge the battery. Instead it will power the load directly. Now, in my test, load current is 6A. The charge current is 5A. So, 1A is drawing from the battery. Compared with no charging test, the voltage dropping rate becomes slow with charging.
 

Irving

Joined Jan 30, 2016
1,863
Thanks for your suggestion. My load testing time is maximum 2 hours only. So, I want to maintain the battery power within 2 hours. If the battery is fully charged, the charger won't charge the battery. Instead it will power the load directly. Now, in my test, load current is 6A. The charge current is 5A. So, 1A is drawing from the battery. Compared with no charging test, the voltage dropping rate becomes slow with charging.
Is that test representative of your real situation? Given you were talking about 12 - 13A and saying the battery wouldn't run long enough... Is 2 hours the acrtual time you need battery support, or just this test? I'm confused as to what your final requirement is.
 

Thread Starter

Yin Min

Joined Jun 24, 2019
27
Hi Irving,

Sorry for my late reply. At first, I was planning to test with 12-13 A load current. But my battery wouldn't run long enough as you explained above. So, I reduced my load current 6A and tested in shorter time.

Thank you,
Yin Min
 
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