Solar charging a LiFePO4 Battery (advice welcome!)

[MikeGray]

Hi folks,
I've been using a LiFePO4 battery for several months to run the starter and electrics on my motorcycle. Although its been working as advertised, I find it much too picky for that job and will be changing back to a traditional SLA. Now I'm looking for an alternative use for this battery. If I could solar charge it, I think it would make a great off-grid camping battery to run a couple LED lights and a fan.

Now, I'm a newb at electronics and could use some advice on how to set this up. Here's what I know about this battery:

Its an X2 Power CYL10039 from Batteries Plus Bulbs

The sticker claims 12v 20Ah Pb Eq, 72.0Wh LiFePO4

The label on the back gives a maximum charge current of 6.6A, and a Min/Max charge voltage of 14.6/15V.

I've been using a Battery Tender specific for LiFePO4 batteries and it charges up to about 14.5V. After sitting overnight the resting voltage seems to be about 13.3v. As long as the temperature is above 40F it seems to happily put out the 270 crank-amps my bike needs to start. Though that won't matter once I change it out.

Unfortunately, I don't know what kind of cells are in this battery, or how many, or in what configuration. Here is my best guess without tearing the thing open. A fully charged bank of cells in series would be around 4.4v, yes? so mine sitting at 13.3v should have 3 banks in parallel. Doesn't tell me how many cells per bank, but from what I've read, a lot of Lithium chargers are set up to charge 3 banks. If this is true, I need a controller that will allow up to 6.6A to charge the battery. Does it matter how much capacity the battery has? A larger capacity would just take longer, right? The controllers I've seen so far are either 3A or 10A. I haven't even looked at solar panels yet.

So the questions I have at the moment are:
- Do I know enough about this battery to pick out a compatible solar controller?
- Do you all have any recommendations as to where to get a controller and what kind or brand?
- Are my assumptions above anywhere near correct?

Thanks everyone for your time and responses.

 

[MikeGray]

Ok, to further confuse the issue, I think I got my series and parallel terms mixed up. Again, electronics newb...

 

[MikeGray]

I've come across some larger LiFePO4 cells that are only 3.2v. So my battery might have 4 cells in it. Still not sure it matters how many cells I have as long as the voltage is right.

I'm thinking of using this controller for it: https://smile.amazon.com/dp/B01N9BP...TF8&colid=2PNK8X8OP06KO&coliid=I22WKEMTGLL1Z9

 

[(*steve*)]

Lithium batteries require chargers which handle the balancing of cells (or at least to prevent overcharging of cells)

I'm not sure how or where the battery management is done (presumably in the charger, but I'm not sure - given that it's a lead acid replacement and is charged by a lead acid charging system???).

Without knowing more about this it's pretty much impossible to say anything. And that is probably why you've not received any answers.

 

[MikeGray]

Thanks Steve, for responding. As stated above, It uses a Battery Tender made specifically for LiFePO4 batteries. I would like to include more info, but neither the Batteries Plus store, nor their headquarters seems to know anything about their own product. I foolishly assumed I'd be able to find any info I needed online, but so far all the other folks I've found who have also bought this particular battery are just as frustrated by lack of info as I am.

I suppose once I get the SLA replacement for my starter I could just cut the sucker open and see what's inside. Right now though, I'm depending on it to start my bike which is my only transportation for the next couple months.

Please ask questions though. As I said, I'm a newb at this stuff so although I've tried to include all the info I have, I may have missed something.

 

[(*steve*)]

It uses a Battery Tender made specifically for LiFePO4

Yeah, but I don't know what that is either.

Does it just connect to 2 terminals on the battery, or does it have a special multi-way connector?

 

[MikeGray]

Just the two battery terminals. No special connectors.

 

[(*steve*)]

OK, in that case it is probably current limited voltage source.

If you have a multimeter, the voltage across the output terminals when disconnected should be the maximum charge voltage for the battery.

That's generally 3.6V per cell for LiFePO4, so I expect your battery has 4 sets of cells in series to give a fully charged open circuit voltage of about 14.4V, and a fully discharged voltage of about 11.2V.

Your charger probably puts out 14.4V with a current limit of some sensible value.

The absolute max for LiFePO4 cells is 4.2V, although the recommended charge voltage is between 3.4 and 3.8. Once charges, they'll settle to about 3.33V or thereabouts.

OK, that corresponds to what your battery says and the measurements you've taken.

This implies that the battery itself contains some sort of balancing for the cells

In that case, you can directly charge the battery from a solar panel provided you do the following:

1. Ensure that the max current from the solar panels does not exceed the max charge current.
2. Ensure the max voltage from the solar panels does not rise above the max charge voltage.

The first of these is easy. Solar panels are generally rated with a max short circuit current. Make sure that is below 6A.

The second part is slightly harder. you can use either a series or a shunt regulator. A shunt regulator has the advantage of being most efficient in terms of getting power to the load, but will consume large amounts of power from the panel when the battery is fully charged (I recommend these regulators for charging at low currents though).

A series regulator will consume more power when charging, and wastes a little power when doing so, but in this case it is probably the best. There are three types of regulators, linear, switch mode, and MPPT.

Switch-mode appears to be the best choice, but in this application it isn't.

A simple linear regulator would be several LM317's in parallel, well heatsinked, and set for 13.5V (because it will be float charging the battery). As long as the voltage of the solar panel under load is not too high, these will not require extraordinary amounts of heatsinking. You would also want to connect the charger to the battery via a Schottky diode and a fuse.

The next option is a MPPT (maximum power point tracking) regulator. These are especially good for solar, but unfortunately are most often set up for a standard lead-acid battery. If you can find one where you can set the output voltage, then this will be the most efficient way to charge the battery.

 

[MikeGray]

Thanks again Steve, this is all very useful! As I read more about this stuff, I'm getting the idea that lithium ion cells are generally around 3.7v whereas lithium iron phosphate cells are 3.2v. Is this correct?

I've continued to search for info about this particular battery I have. Came across charging instructions:
As you suspected, there is a built-in protection circuit module which provides over-voltage, over-current, temperature protection, and even charge balancing.
The battery is compatible with 12v smart lead-acid chargers so long as they don't have a desulphation mode. Also, the lead-acid charger would need to be disconnected once it enters the float-voltage stage.
So that explains how it is able to be a direct replacement. Too bad the folks who sold it to me didn't know any of this! I'll stick with my lithium charger though, since it has the specific charging stages the battery wants and does it all automatically.

There is a lithium solar charge controller that I think will work. Its a Genasun GV-5L, 5 Amp 12 Volt MPPT Solar Charge Controller It allows the battery to charge up to 14.2v before reducing the current. Is that close enough to 14.4v? It also has a normal current output of 5A, but says it could peak at 9A. I think it depends on what the solar panel is doing. I still have to learn about panels, but from what I've read about MPPT controllers, they can actually tell the panel how many Volts and Amps to produce. I will need to find a way to make sure the current doesn't go over 6A.

So here's yet another dumb newb question: Since this battery will be following behind my motorcycle anyway, I wonder if I can also keep it charged from the alternator. I know some folks have an accessory battery on their vehicles hooked up to an isolator so the starter only uses power from the starter battery and the accessories only use power from the accessory battery, yet both are charged by the engine's alternator. Would I need to keep the two systems separate? The bike's alternator puts out more than enough power, and the battery is already set up to charge off of it. I wonder though if that would interfere with the solar charge controller somehow?

 

[(*steve*)]

Charging from the alternator will possibly be ok, but ensure you isolate the batteries using a diode.

To batteries of the same type in parallel are generally ok, but if they have different chemistries as they do in this case can easily lead to one battery discharging into the other.

An additional diode drop to the lithium cell should reduce this significantly. However, given the relatively low charge current means you could possibly charge from accessory power (still via a diode) so the battery is fully isolated once you remove the keys from the ignition.

 

[MikeGray]

I found an isolator for the bike that looks promising. Its rated for 75A (100A peak) which is more than enough for my 40A alternator. It also gives priority to the starter battery before the accessory one, And its fairly compact so I should be able to find a mounting point pretty easy.

I've got some more reading to do about solar panels, but I'm almost ready to start getting some parts and putting stuff together!

 

[AprilSteel]

This thread is a bit old but my comments are pertinent to this technology .

I have been playing with these BMS units below lately but it looks like yours might have a BMS built in? If it does any battery charger will do as it will be cut off when full at about 3.55V per cell or 14.2Volts and it won't go below a set voltage when discharged , typically 10 Volts for that tech
https://www.ebay.com.au/itm/4S-w-Balance-100A-3-2V-LiFePo4-LiFe-18650-Battery-Cell-PCB-BMS-Protection-Board/401520816753?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

 

[AprilSteel]

I've come across some larger LiFePO4 cells that are only 3.2v. So my battery might have 4 cells in it. Still not sure it matters how many cells I have as long as the voltage is right.

I'm thinking of using this controller for it: https://smile.amazon.com/dp/B01N9BP...TF8&colid=2PNK8X8OP06KO&coliid=I22WKEMTGLL1Z9

My understanding is that LiFePo4 technology is nominal 3.2 Volt . Max about 3.6 and min about 2.5V. Your battery will have 4 cells in series inside .
See the graph ,it is a very flat discharge/charge curve.

I have noticed also that many of the sellers do not understand the different voltages of LiFePo4 as opposed to LiPo4 /Lithium Ion which is a nominal 3.7Volt technology and has higher charge discharge curves. Your controller does not give these limits that I can see so I would be careful using it.

 

[MikeGray]

Thanks April,
I've learned tons more about both Li-Ion and LiFePO4 since starting this post. I'm still using this same battery to start my motorcycle, so I have yet to crack it open. But I do know that it has an internal BMS which keeps the alternator from damaging it.

It's been working pretty well, but seems to discharge if the bike is left for more than a couple days without riding. It's way worse in the winter, but I suspect I have a parasitic draw somewhere that I'll need to troubleshoot.

Also, from what I've learned, I'm going to have to open the battery box up in order to find out the brand and model of cells before I can learn full specs on them. But with an internal BMS, It makes sense that any 12V battery charger less than 6A would work as long as it doesn't have the "desulfation" feature.

I have noticed that on occasion, my LiFePO4 battery tender will charge the battery to full, and then it thinks the battery is still full after it has slowly discharged. I don't know if that's an issue between the charger and the BMS, or if the charger itself just needs to be reset every now and then. Not a huge deal though, now I know it happens. Just have to remember to unplug it once in a while during winter.

 

[AprilSteel]

All good Mike . I doubt you need to know what cells are in there. There will be 4 . Probably Your charger will charge them up to say 3.5 Volts and then it will have a cut back in voltage lower than that which might be say 3.4 Volts so until the battery drops to that it will not recharge. Some of these have a very flat discharge curve so it could be a while on a used battery before it gets down to that figure .

I would probably put a multimeter on it from time to time especially if you think it is discharging too low. You may find its still in the useable range because they have a much lower DOD (depth of discharge) than lead acid . At 2 Volts per cell or 8 volts on the pack some manufacturers say they are still good . personally I use 10 volts as a low point.

Supposedly the LIFePo4 like the colder weather rather than hot engine bays so maybe you do have a problem there .

Anyway they are still going so they have beaten lead acid by the sound of it . Cheers

 

[hardwaylearn]

All good Mike . I doubt you need to know what cells are in there. There will be 4 . Probably Your charger will charge them up to say 3.5 Volts and then it will have a cut back in voltage lower than that which might be say 3.4 Volts so until the battery drops to that it will not recharge. Some of these have a very flat discharge curve so it could be a while on a used battery before it gets down to that figure .

I would probably put a multimeter on it from time to time especially if you think it is discharging too low. You may find its still in the useable range because they have a much lower DOD (depth of discharge) than lead acid . At 2 Volts per cell or 8 volts on the pack some manufacturers say they are still good . personally I use 10 volts as a low point.

Supposedly the LIFePo4 like the colder weather rather than hot engine bays so maybe you do have a problem there .

Anyway they are still going so they have beaten lead acid by the sound of it . Cheers

 

[hardwaylearn]

Lifepo4 batts can be treated like leadacid. I bought a 12v jump starter , it was dead flat from the shop. Would discharge internally overnight. Balance circuitry was faulty, so I removed it . Then I joined the 4 cells in series to make 14v batt. It is charged by constant volt charger at 14v watching the ampmeter, when rate is zero batt is fully charged. It is 3.5v per cell. It now works as jumper like it should. I give top up charge as needed maybe 3 timesa year.

 

[AprilSteel]

Lifepo4 batts can be treated like leadacid.

No they can't Mate. The main reason being LFP needs to be charged to 3.65V per cell only on the first charge . Thereafter stay at a max of 3.5V because if you go over you will be damaging the SEI layer and will eventually choke the battery. Thats why your phone ,laptop and cordless batteries are dying early . The chargers that come with them are made to charge to top voltage each time and it kills them . Turn all these chargers off before they reach full charge.

Above 3.65V per cell and you start laying down metalic lithium and it dies even quicker.
lead acid chargers typically charge higher than that and if equalisation is built in can go to over 15V for lead acid on equalisation or desulphation phase and your LFP is history..

 

[hardwaylearn]

No they can't Mate. The main reason being LFP needs to be charged to 3.65V per cell only on the first charge . Thereafter stay at a max of 3.5V because if you go over you will be damaging the SEI layer and will eventually choke the battery. Thats why your phone ,laptop and cordless batteries are dying early . The chargers that come with them are made to charge to top voltage each time and it kills them . Turn all these chargers off before they reach full charge.

Above 3.65V per cell and you start laying down metalic lithium and it dies even quicker.
lead acid chargers typically charge higher than that and if equalisation is built in can go to over 15V for lead acid on equalisation or desulphation phase and your LFP is history..

I meant charging as stated in my reply only. As to charging lilon cells i have charged hundreds to 4.2v full charge no worries repeatably and used to 2.7v level. These are quality cells made by Sony. Cheap chinese cells are failure prone.

 

[AprilSteel]

No worries mate . Its becoming obvious now that charging LFP , LiPo,LiIon etc it is important not to go back to the full charge . That sets up the SEI layer on first charge and you should not go back there again for fear of clogging the cell. Most lithium chargers are set to the max as you know ,4.2 in LiIon but have you noticed how those batteries treated that way die early ? There is a video on UTube from a Uni Proff called "whats inside a Lithium cell" or something like that . Its a good learn and the SEI layer is in the last third. Not criticising just informing . Cheers