LFP Dropins that actually disconnect the cells once fully charged?

[Jman99]

I have two general questions about using these.

1, if using one of these as the cranking battery under the hood, will the cars electronics be ok when the battery is disconnected from it? Any unforeseen problems? Some online searching yeilds mixed answers.

and 2, does victron solar chargers care about this kind of behaviour if charging the second house battery, say when you have a good few hours of light left running the fridges off solar?

ta

 

[dreadlocks]

They only have a single Charge/Discharge bus by definition, so no... they cant possibly disconnect the cells when full or your whole battery goes dark, you gotta have a charger that drops to a float once charged.

 

[Rando]

The drop in batteries have a single path BMS, as are a lot of the stand alone BMS. The BMS has back to back NMOS FETs, so it can essentially switch in a diode to the circuit going either direction (charge or discharge). If the BMS hits the high voltage cut off, then the charge MOSFET is turned off, and the battery can only discharge and the the inverse for the low voltage cut off. Even if you hit the high voltage cut off, the car can still run off the battery, so no big deal there. Similarly if you run the battery all the way down, the charge MOSFET stay on so that you can charge it even though you can no longer get power out of it.

However, you really shouldn't be hitting your BMS limits. They are there as a safety, but your charge sources should be within these limits.

 

[Jman99]

They only have a single Charge/Discharge bus by definition, so no... they cant possibly disconnect the cells when full or your whole battery goes dark, you gotta have a charger that drops to a float once charged.

I don't buy it myself, but some are saying their dropins actually do this & the loads are carried by the alternator, but just curious.

 

[jonyjoe101]

With a car alternator that puts out 14.4 volts, it should never reach a point for the bms to activate at 14.6 volts, especially while engine is running and using power. The only problem would be if the battery were to go out of balance then the bms will activate, but since the car is running the alternator will have a load that it can output its power to. I don't understand how the voltage regulator works in a car environment but that might be something that prevents voltage surges.

To your number 2 question, with solar charge controller, if the bms activates, that is very bad. You will get voltage surges. Voltage surges can reach over 20 volts which will destroy any 12 volt devices that can't handle such overvoltage. The only solar charge controller that i have seen that actually states that it can handle a bms activating without causing voltage surges is the makeskyblue 60 amp mppt Below is the note on bms activating while connected to solar.

" 3. The advanced application for V118 50A and 60A:If your load device is shorted or over-current, the controller will have output short-circuit, protection and close the output. When you disconnect the load and press ENT over 5 seconds, then output will turn on again. Please note the 30A and 40A don't have this function. The MPPT output of the battery voltage will not rise suddenly, and the controller doesn't cause the output voltage to overshoot through the BMS protection board.
By the way, the charging voltage of V118 version can be set from 12V to 17V compare to the fake products is from 13.5V to 17V. The discharge voltage can be set from 9.8V to 14V.

I always look at controller manuals to check how it reacts to a bms activating, and most manuals don't mention it. I don't know how the victron would react but I would be hesitant to use it on lithium unless I knew for sure how it would behave. I know from reading other forums that battleborns will cause voltage surges when the bms activates on solar, any lithium battery that uses mosfet bms will cause voltage surges if activated while on solar.

 

[Rando]

Have you ever actually observed one of these 'voltage surges'?

I certainly haven't, and have disconnected my battery from my solar controller while active, many times. Victron even suggests putting a battery protect (a MOSFET switch) between the charge controller and the battery bank, so they clearly don't see this as an issue.

Theoretically I guess a super poorly designed charge controller could have this problem, but it is not something to worry about with quality equipment.

With a car alternator that puts out 14.4 volts, it should never reach a point for the bms to activate at 14.6 volts, especially while engine is running and using power. The only problem would be if the battery were to go out of balance then the bms will activate, but since the car is running the alternator will have a load that it can output its power to. I don't understand how the voltage regulator works in a car environment but that might be something that prevents voltage surges.

To your number 2 question, with solar charge controller, if the bms activates, that is very bad. You will get voltage surges. Voltage surges can reach over 20 volts which will destroy any 12 volt devices that can't handle such overvoltage. The only solar charge controller that i have seen that actually states that it can handle a bms activating without causing voltage surges is the makeskyblue 60 amp mppt Below is the note on bms activating while connected to solar.

" 3. The advanced application for V118 50A and 60A:If your load device is shorted or over-current, the controller will have output short-circuit, protection and close the output. When you disconnect the load and press ENT over 5 seconds, then output will turn on again. Please note the 30A and 40A don't have this function. The MPPT output of the battery voltage will not rise suddenly, and the controller doesn't cause the output voltage to overshoot through the BMS protection board.
By the way, the charging voltage of V118 version can be set from 12V to 17V compare to the fake products is from 13.5V to 17V. The discharge voltage can be set from 9.8V to 14V.

I always look at controller manuals to check how it reacts to a bms activating, and most manuals don't mention it. I don't know how the victron would react but I would be hesitant to use it on lithium unless I knew for sure how it would behave. I know from reading other forums that battleborns will cause voltage surges when the bms activates on solar, any lithium battery that uses mosfet bms will cause voltage surges if activated while on solar.

 

[jonyjoe101]

I seen these voltage surges with both pwm and mppt controllers, and read about other people having similar problems with differnt brands of controllers. I always have a 12 volt swampcooler running and have lost numerous fans and pumps from the surges. This only occurs with the battery connected to controller and the bms activated.

When the mosfets activate they leak voltage. The controller reads a battery at 11.7 volts and tries to charge it. The battery wont accept current, as the controller keeps trying to charge it, you get voltage surges. When you disconnect the battery from controller, nothing happens since the controller reads 0 volts. I wouldn't recommend using a mosfet switch, since it also leaks voltage. I tried using an overvoltage relay to mechanically disconnect the battery if voltage got to high, but they respond too slowly, equipment is damaged by the time they disconnect the battery.

If you always undercharge your lithium and its in perfect balance, you will never activate the bms. I bought the makeskyblue 60A controller but I havent tested it in "anger" to see if it will actually do what it claims. For me an actual test if the only proof I would accept.

 

[Jman99]

Thanks jonyjoe, If I understand Rando the BMS can change the direction of the "diode", so it doesn't really completley disconnect the cells, just blocks charging, so no issue with the mppt or alternator.

 

[jonyjoe101]

Actually from what I read on other forums, when the bms activates, the battery won't charge or discharge. I think everything has to be disconnected so it resets. In my situations when it activates, my fans,lights everything go off, I have to unplug the battery and then reconnect and apply a load to get it to restart. Sometimes it starts right away, other times it takes a while if the bms still reads a high cell or out of balance situation.

If a alternator is being used, at 14.4 volts the battery will never get fully charge, I've connected my 220ah lifepo4 battery to the alternator system and I barely see 2 to 3 amps going into the battery. You have to force the amps into the battery and for that you need more then 14.4 volts.

With mppt you have issues if you want to fast charge the battery. To fast charge you will have to bulk charge higher then 14.4 volts. With mppt controllers due to the thinner gauge wire use, there might be voltage drop from controller to battery. On my 220ah lifepo4, if I set the bulk to 14.6 volts, my battery charges at 4 to 5 amps, I need to raise the bulk higher to get max amps into the battery. To get max amps from my 240 watt panel I have the bulk setting set to 15.5 volts.
At 15.5 volts on the controller, the battery terminals barely reach 14.6 volts, thats what voltage drop will do. I prefer charging at 15 amps (shown on picture) versus 4 or 5 amps.

I don't think the alternator or the mppt controller (set to 14.4 volts) will ever give a lifepo4 a full charge, maybe 90 percent. The only way to fully charge would be slow charging with a buck converter or the special chargers they sell for those batteries. I can charge to 100 percent but I use active balancers and a bms that uses mechanical relays.