Hi All,
The Intro
I regularly get asked by customers, potential customers and other forum members about what I recommend for charging of LiFePO4 cells. Rather than repeating myself every time I thought I'd post what I think is right and get some debate going!
I have a ton of research, tests and practical experience in this area. I think now have a methodology for keeping these cells safe down to a tittle.
To summarise the rest of this post. No single cell should touch the top end of the charge curve, ever. No single cell should touch the bottom of the charge curve, but if it does, all other cells should do so at the same time.
The below graph is of a single Sinopoly 60AH (B) cell being discharged at a little over 30A (0.5c) measured capacity was exactly 60AH. This cell had been over-discharged and abused in the past so probably not the best example! They normally come in closer to 70AH.
Note how the voltage spikes at the start and begins to fall out rapidly at the end.
I'm a firm believer that *most* BMS usually do more harm than good, and cost a bloody fortune! The only time I can see any use for them is when you need to squeeze every AH out of the pack. generally, I'd say spend the money you'd of spent on a BMS on more cells.
I think the method Jack Rickard (EVTV) often describes is more or less spot on. I just want to elaborate on this and add some extra thoughts.
Identifying your overall voltage and cell drift.
I would recommend bottom balancing each individual cell (once) to 2.70v and then charging in series until any single cell reaches 3.5v (identifying it with a normal multimeter by running around the pack). The overall voltage when that first cell reaches 3.5v then becomes your charge cutoff voltage for all subsequent charges. You just need to check that that cell (the weakest) every few hundred cycles to see if it is still the first to reach 3.5v (in case another cell becomes your weakest as they age differently). And also re-program your charge cutoff voltage.
All loads on the cells (charge and discharge) should be equal. (The following will be controversial!) Cell drift doesn't exist, except for unequal losses in capacity over time. So adding unequal loads such as a BMS, or voltage sensing (split pack method etc.) means that cells become out of balance, very quickly.
In short, the only wires on the cells should be to connect to the cell next to it and at either end of the pack to connect to the HV components. Nothing else. The whole pack needs to be treated as one.
Why bottom balance?
By bottom balancing you are protected for over-discharge as all cells will reach zero capacity at more or less the same time meaning no single cell will go into reversal (at least not significantly with hundreds of amps passing through it, none of the other cells should have enough charge to allow that to happen). I'm pretty sure everyone will over-discharge their cells at some point, and without a bottom balance you could be looking at dead cells and possibly worse!
I have successfully brought back all cells in a 4 cell bottom balanced pack that went down to less than 0.1v. No noticeable losses in capacity.
By cutting of the charge when the first cell reaches 3.5v (or higher depending on your risk appetite), you ensure that your weakest cells (the lowest in capacity) never get overcharged.
Charge Cutoff
I would recommend having your charger set to cutoff when at the voltage determined above. And then using something like a JLD404 as backup in case the charger fails (voltage sensing etc.).
The JLD404 has some really good features (AH counting, programmable relays etc.) You can use one of the relays to trigger the BMS or EOC (end of charge) control on your charger (if available) at just above (as in 0.5v above) the charge cutoff voltage determined above. This ensures safe shutdown of the charger if something goes wrong.
You could then use a second relay output on the JLD at say 1.0v above the pre-determined voltage to trigger an appropriately sized AC relay/contactor on the chargers input. Just in case the other two systems (above) fail.
Using a latching relay tied in to both of the above relays you can set up some kind of alerting system (beep, LED etc.) the next time you turn the key. This will let you know that the JLD has had to intervene, allowing you to investigate.
Monitoring and cell loading
Finally, you just need to keep monitoring the pack as a whole. If you get in your car and realise the voltage is higher than normal or an extra 5AH has gone in (or not gone in!), something isn't right and needs investigating. The JLD404 can monitor all of this.
These prismatic LiFePO4 cells are really robust and age very slowly when treated right. The characteristics of a single cell will change very slowly as it is cycled and time goes by (2-3,000 cycles to 80% of its original capacity is about right when treated as above).
As previously mentioned. If the cells are all equally loaded (exactly) and treated as one they will not drift apart. They will however age differently depending on things like environmental conditions (one cell next to a heat source will age slightly differently to one that isn't, slight differences in production etc.)
This is why it is worth running around with the multimeter every few hundred cycles just to ensure that your weakest cell (the first to reach 3.5v) is still your weakest and then re-setting your cutoff voltage. If that cell is no longer he weakest you need to find out why and go through the above process again. If anything ever goes wrong (it shouldn't), re-bottom balance and start again.
Hope this helps and let the debate begin!
Cheers,
Mike
The Intro
I regularly get asked by customers, potential customers and other forum members about what I recommend for charging of LiFePO4 cells. Rather than repeating myself every time I thought I'd post what I think is right and get some debate going!
I have a ton of research, tests and practical experience in this area. I think now have a methodology for keeping these cells safe down to a tittle.
To summarise the rest of this post. No single cell should touch the top end of the charge curve, ever. No single cell should touch the bottom of the charge curve, but if it does, all other cells should do so at the same time.
The below graph is of a single Sinopoly 60AH (B) cell being discharged at a little over 30A (0.5c) measured capacity was exactly 60AH. This cell had been over-discharged and abused in the past so probably not the best example! They normally come in closer to 70AH.
Note how the voltage spikes at the start and begins to fall out rapidly at the end.
I'm a firm believer that *most* BMS usually do more harm than good, and cost a bloody fortune! The only time I can see any use for them is when you need to squeeze every AH out of the pack. generally, I'd say spend the money you'd of spent on a BMS on more cells.
I think the method Jack Rickard (EVTV) often describes is more or less spot on. I just want to elaborate on this and add some extra thoughts.
Identifying your overall voltage and cell drift.
I would recommend bottom balancing each individual cell (once) to 2.70v and then charging in series until any single cell reaches 3.5v (identifying it with a normal multimeter by running around the pack). The overall voltage when that first cell reaches 3.5v then becomes your charge cutoff voltage for all subsequent charges. You just need to check that that cell (the weakest) every few hundred cycles to see if it is still the first to reach 3.5v (in case another cell becomes your weakest as they age differently). And also re-program your charge cutoff voltage.
All loads on the cells (charge and discharge) should be equal. (The following will be controversial!) Cell drift doesn't exist, except for unequal losses in capacity over time. So adding unequal loads such as a BMS, or voltage sensing (split pack method etc.) means that cells become out of balance, very quickly.
In short, the only wires on the cells should be to connect to the cell next to it and at either end of the pack to connect to the HV components. Nothing else. The whole pack needs to be treated as one.
Why bottom balance?
By bottom balancing you are protected for over-discharge as all cells will reach zero capacity at more or less the same time meaning no single cell will go into reversal (at least not significantly with hundreds of amps passing through it, none of the other cells should have enough charge to allow that to happen). I'm pretty sure everyone will over-discharge their cells at some point, and without a bottom balance you could be looking at dead cells and possibly worse!
I have successfully brought back all cells in a 4 cell bottom balanced pack that went down to less than 0.1v. No noticeable losses in capacity.
By cutting of the charge when the first cell reaches 3.5v (or higher depending on your risk appetite), you ensure that your weakest cells (the lowest in capacity) never get overcharged.
Charge Cutoff
I would recommend having your charger set to cutoff when at the voltage determined above. And then using something like a JLD404 as backup in case the charger fails (voltage sensing etc.).
The JLD404 has some really good features (AH counting, programmable relays etc.) You can use one of the relays to trigger the BMS or EOC (end of charge) control on your charger (if available) at just above (as in 0.5v above) the charge cutoff voltage determined above. This ensures safe shutdown of the charger if something goes wrong.
You could then use a second relay output on the JLD at say 1.0v above the pre-determined voltage to trigger an appropriately sized AC relay/contactor on the chargers input. Just in case the other two systems (above) fail.
Using a latching relay tied in to both of the above relays you can set up some kind of alerting system (beep, LED etc.) the next time you turn the key. This will let you know that the JLD has had to intervene, allowing you to investigate.
Monitoring and cell loading
Finally, you just need to keep monitoring the pack as a whole. If you get in your car and realise the voltage is higher than normal or an extra 5AH has gone in (or not gone in!), something isn't right and needs investigating. The JLD404 can monitor all of this.
These prismatic LiFePO4 cells are really robust and age very slowly when treated right. The characteristics of a single cell will change very slowly as it is cycled and time goes by (2-3,000 cycles to 80% of its original capacity is about right when treated as above).
As previously mentioned. If the cells are all equally loaded (exactly) and treated as one they will not drift apart. They will however age differently depending on things like environmental conditions (one cell next to a heat source will age slightly differently to one that isn't, slight differences in production etc.)
This is why it is worth running around with the multimeter every few hundred cycles just to ensure that your weakest cell (the first to reach 3.5v) is still your weakest and then re-setting your cutoff voltage. If that cell is no longer he weakest you need to find out why and go through the above process again. If anything ever goes wrong (it shouldn't), re-bottom balance and start again.
Hope this helps and let the debate begin!
Cheers,
Mike