charging LiFePO4 battery in car - am I on the right track

[shmick82 Junior Member Find all posts View Profile Close shmick82]

Hi,
Im am going to install a LiPo as a second battery in the car. Regarding charging what I understand is never to go too low (10.5V) and not too high (16.8V) (those figures are from seller and or manufacturer).
Since I already have an electronic isolator from previous setup, it starts charging the second battery (the LiPo) when voltage is at 13.4V and cuts out at 12.8V. I am making the assumption the alternator on the car (120 Grande) doesnt go over 15 (or 14.4?)
Therefore the charging range, via the isolator, is 12.8V-14.4V, which is a good range to charge the LiPo battery. So I don't need any other devices.
Am I missing something or is it basically correct.
NOTE: The isolator is connected to 50A anderson via 6B&S cables to boot of car, with 2 circuit breakers in between.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

The 16.8V seems rather high for a LIFePO4, I thought it was around 14.6V from memory?

A normal low voltage cut should protect the battery from over discharge, the upper voltage limit is the one you need to be careful of as overcharging the battery can result in a catastrophic failure.

Most manufactures recommend some sort of battery management system but I have read where others are using these batteries and write that cell balancers and over voltage protection aren't required.

If your building the battery yourself, personnaly at this time I would go for a cell balancing system that attaches directly to each cell and some sort of overvoltage protection to be on the safe side. A simple unit that isolates the battery if the voltage gets to high.

Your 120 is unlikley to get above 15V unless you visit the snow but a regulator failure for instance could cause very hig charge voltages so would be better to be safe than sorry with the price of the Lithiums as they are.

[shmick82 Junior Member Find all posts View Profile Close shmick82]

Thanks LeighW,
The cell is already built, any recommendations for high voltage cut offs? That are cost effective, like some sort of relay, or circuit breaker? I notice Redarc has some dedicated chargers, they cost as much as the battery.

[Symo Advanced Member Find all posts View Profile Close Symo]

I replied to your other thread, but BE CAREFUL. LiPo are unstable and when (not if) they fail, they do so with fire. Do you want that in your car?

Plus what Leigh said, the LiPo will be made of 3.2v cells and all cells need to be balanced the same (or fire) so you need a cell balancing charger which will be DC-DC (we use them at work). You can't use your alternator (or fire).

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

LIFePO4 batteries don't pose the same fire risk as the old type, they are supposedly
safe though overvoltage/charging will cause catastrophic failure ie the will crack open.

There are now types made that are direct drop in replacements for lead acid batteries
that appear to have a built in battery management system.

Shmick82,

A visit to one of the Lithium specialists ie EV works, they should have an overvoltage
cutout etc though any such devcie as long as you can set the voltage should do. Current handling should not be an issue as you can always add a suitable relay into the circuit.

As long as the charge voltage is in the range specified you shouldn't need a dedicated charger, well according to some battery specs but then there is some much contradicting imformation it is very to what is and is not required. I would be wary of any charger unless you know it has a proven track record, I recently tested a unit by a supposed very well respected company and found it would have not done any battery including Lithiums any good!

Some say as long as the cells are well balanced and matched to start with you don't need cell balancers, others say just monitor during charging is sufficient, others you must have cell balancers and chargers.

It really is the dark arts at the moment, from what I have read again the only real issue I see is the overvoltage issues, I'm wait for a while to see what the feedback is regarding the drop in units, many are using these to replace the cranking battery and the feedback I have recieved from a couple of users is the work great with no problems so far.

I have been tempted to try the 100AH drop in replacement units but have read negative reports by some regarding the company concerned but the bun fights you see on different forums are worse than the VSR versus DCDC chargers debates.

Unfortunatley it is the usual I have this system and it is the only way to do it, if you don't do it the way I have it setup you will stuff your batteries.

Personnally I think I'll just get a couple of 100Ah units down the track, I'll put the type of cell balancers that bolt directly to the cell terminals and an over voltage cut out and charge straight off the alternator, then again maybe a drop in unit

[Corigator Avid PP Poster! Find all posts View Profile Close Corigator]

The 16.8V seems rather high for a LIFePO4, I thought it was around 14.6V from memory?

So Leigh,

Are you saying that if we go for a LiFePo4 battery we should ditch your diode?

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Personnaly I wouldn't as it is unlikely you would see 14.6V at the battery, I would
just put a high voltage cut out in to bullet proof it should you get a regualtor
failure etc.

If the battery is marketed as a drop in rpelacement for a lead acid then it should be
able to handle whatever the alternator can normally output anyway, in very cold climates
the alternator could be charging at 15V or more. One would therefore have to assume the
battery has some sort of inbuitl battery management system?

[t303 Senior Member Find all posts View Profile Close t303]

I have two separate hivolt alarms (turbo timer and Madman EMS) that will both be screaming their t*ts off if it goes high.
IMHO battery management systems are not required unless you are trying to push the limits on charge and discharge voltages. If you stay in the middle of the bell curve you are unlikely to have any problems. I check individual cell voltages (out of idle curiosity) occasionally and rarely have more than a couple of tenths of a volt difference between cells. That is another advantage of the Li cell setup. If you were able to measure the individual cells in an AGM you would probably be somewhat alarmed at the different states of each, but you never know how they are until the cell fails (as has been the failure mode of just about every LA battery I have owned). Also there have been cases of BMS modules killing cells due fault. They work by bleeding off (via resistor) any excess voltage on a single cell. Because they are an electronic device and able to fail, they may "lock in" and drain the cell completely while you aren't watching. For charging, I just use the GEL setting on my charger which gives 14.1v; well within the "safe" range where the chance of cell damage is unlikely. IIRC "full" for LiFe is 14.4v, and the difference would be a few % of capacity.

[t303 Senior Member Find all posts View Profile Close t303]

Just following on about charging methods and capacity, it really is a different ball game from LA battery tech.
Where LA needs to be taken to 14.4v and held there for an amount of time to get that last bit of capacity in, Li will get it's capacity regardless of the voltage. Simply put, if you think of the battery as a bottle of energy, to get a full bottle of energy from AGM you need to pressure extra in to enable it to get to full, with Li you don't need the extra pressure (time, energy, however you want to think of it), you just have to tip it in to fill it up. Not a very scientific explanation but I hope it is clear.

[shmick82 Junior Member Find all posts View Profile Close shmick82]

FYI-by LiPo i menat LiFePO4, sorry. Thanks for all the replies

[stevehastings Junior Member Find all posts View Profile Close stevehastings]

mmmm interesting, I use a lot off lipos, most 4cells. I have a power poilt cell balancer working along side one of my chargers that stops charging at 4.20v per cell. My other charger has a balancer built in to it. Never let the cell drop below 3.8v, it will shorten the life and never keep them full power for an extended time with out use. When they start to PUFF UP, time to get new lipos, or risk fire. I have had one fire in the last 5 years. Cheers Steve.