LiFePo4 and Solar Charging.

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juanito
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LiFePo4 and Solar Charging.

Hi. I've been working on a small aluminium frame NEV that i was wanting to add a large 500W PV array on the top to charge the batteries and help with powering the 1KW rated motor. The battery pack is a 20Ah 48V LiFePo4 with it's own PCM. However I can't seem to find a solar charge controller for LiFePo4 batteries. I'm affraid of using a charge controller designed for lead-acid and NiMh batteries although I have found some that have adjustable bulk (charging) and float voltages that would be conducive to LiFePo4 batteries. But they always seem to have long periods (i.e. 1 hour) in which the batteries are held at bulk voltage after they reach the bulk voltage. Would this damage LiFePo4 batteries? Are there any solar charge controllers for 48v LiFePo4 battery systems out there?

dogman
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Re: LiFePo4 and Solar Charging.

Most of the lifepo4 pack chargers out there are simply sla chargers with the voltage adjusted slightly up to engage the balancing function of the bms. As long as the charge controller is 48v, I don't see why it wouldn't work. Even if it didn't balance, you won't need to balance every single charge anyway. You could always just plug in to balance the pack from time to time. The long period at bulk voltage will most likely help the batttey balance. The bms will kick in if any cells get overcharged, so I don't see how it could hurt the pack.

Be the pack leader.
36 volt sla schwinn beach cruiser
36 volt lifepo4 mongoose mtb
24 volt sla + nicad EV Global

juanito
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Re: LiFePo4 and Solar Charging.

Thank you very much! That was great help!

juanito
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Re: LiFePo4 and Solar Charging.

So let me get this strait. The max recomended charging voltage for a single cell is 3.65V (correct?). So 16 in a series would be 58.4V (which is also recomended on the manual for the battery pack). (According to the charge controller's manual, SLA batteries should be set at 56.4V.) So if I set my solar charge controller's bulk voltage to 58.4V it should be fine. From what I understand, the balancing function starts balancing each cell above 3.63V so the voltage should be more than 58.08 for charging the pack anyway. After it reaches the bulk voltage of 58.4V and stays there for an hour that would give it time to balance. If I got the right info for this PCM it will start an overcharge recovery for an individual cell at or above 3.8V and will cut off the whole pack if a single cell reaches 3.95V for any reason. After the hour at bulk charging the controller goes to trickle charge. This I could set at maybe 53.2V (adjustable every .8V) since That would be close to an idle voltage of 3.3V per cell. (Controller's manual recomends 54.4V for SLA batteries trickle charge.) I don't have to worry about regulating the amperage since the solar panel will have a nominal amperage about the same as the recomended charge rate (2 x 3.5A in parallel = 7A with 180 x .5V in series = 90V or maybe 3 x 3.5A in parallel = 10.5A and 120 x .5V = 60V).

dogman
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Re: LiFePo4 and Solar Charging.

Sounds to me like you have a handle on it. Of course, if your pack doesn't need balancing, you could set it lower. The way I am using my pingbattery, moderate rates of discharge and moderate depth of discharge, I rarely see my charger do any balancing. It won't go into balancing mode unless it is out of balance which is seldom. You could always just use the regular charger occasionally for a balancing charge.

Be the pack leader.
36 volt sla schwinn beach cruiser
36 volt lifepo4 mongoose mtb
24 volt sla + nicad EV Global

juanito
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Re: LiFePo4 and Solar Charging.

I had another crazy idea. I could limit my photovoltaic cell strings of 100 which would put out a nominal voltage of 50V (10.5 amps) and limit the open circuit voltage to about 60V (0 amps) The strings could be protected with a few Schottky diodes. With this option I could use prismatic cells like a couple of Ping's batteries, and charge from the main cables that go to the controller (charging it like it would in a regenerative braking situation) with no charge controller.

With this set up I could run right off the solar panel. The only problem is that under full throttle without the battery pack turned "on" the motor would draw the voltage down and therefor acceleration and top speed would be less. I estimate that it wouldn't be able to drop below the 18V limit of my controller (unless cloudy). But if not, I could allways take it easy on the throttle.

I asked Ping if the 48V 20AH packs could be trickle charged. I was told that a trickle charge voltage, that is, a voltage lower than 60V-61V would ruin the BMS. (I wonder why and if it has to do with reverse current through the charging circuit that could be fixed with a diode.) But I was also told that they could be regen charged for no more than 5 amps per battery pack. That's why I decided to investigate charging them a differnt way since weather could affect charging voltage.

Charging amps for 2 packs should be about 5.25A per pack (a little high) on a sunny day slowly dropping down to 0 as it nears the open circuit voltage (58V-60V). This may mean they'll never fully charge, but like Dogman said, it may be a good idea to charge them up at home over night from time to time so they can balance. I just hope this won't hurt the BMS's.

Efried
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Re: LiFePo4 and Solar Charging.

I'm doing an experiment using two 12V panels attached to 12 cells or less in order to maximise the yield.
When it is a dull day I'm connecting less cells accounting for the lower PV-voltages. I'm also switching the connected cells.
Since the battery is 50Ah and the maximum measured PV-power only 19W I hope not to damage the battery if I do this for a partially charged battery.
Cell voltage measurements are to follow.

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