Solar Freaks

Maybe some of you on this forum can help with a problem I've been having: occasional battery overcharging. My knowledge of electronics is limited. Commercial controllers I've seen on the internet are very expensive, and seem overkill for my purposes.

I use an array of 6 60W PV panels to charge LiFePO4 batteries, mostly e-bike battery packs of 16 or 20 series-connected cells.

Is there a simple, reliable, and inexpensive way to throw a switch when voltage climbs to a settable point? If so, I would use 4 panels in series to charge 16-cell packs, with shut-off at 60V; for 20-cell packs, 5 panels in series, with shut-off at 75V. These parameters would put the panels at near their MPP, preclude overcharging, and charge in a timely fashion.

Any help would be much appreciated!

This is the way I have been doing things:

I use 4 panels in series to charge 20 cell packs; in cooler weather, 3 panels in series will charge 16 cell packs. Batteries can be fully charged this way, but not become overcharged, even if left attached for some time; when the packs are fully charged, the panels are so near Voc that little current flows. However, the time to achieve full charge is relatively long, as the panels are far from their MPP.

During the hot summers here in the high desert, 16 cell packs cannot be fully charged with 3 panels in series, so it's necessary to use 4 panels in series, and watch charging closely. On a few occasions I was not attentive enough, resulting in batteries charged to higher than recommended voltages.

(See attached graphs - it may be necessary to click on them for best view. A pack can be considered fully charged when the component cells reach 3.7 - 3.8 volts, and can be taken a bit above 4 volts without damage.)

The first thing about charging lithium batteries is to very carefully monitor the cell voltage. For instance, a 10 mV overvoltage significantly reduces the lifetime of the cell. At 30 mV over, the cell may catch fire (the membrane inside breaks down over time and much more rapidly when over voltage).

The different chemistries, Li-Ion, LiPo, LiFePo4, have different cell voltages. I'm assuming you've read up on constant-current charging, followed by constant-voltage charging, and continuous temperature monitoring? Those three limits are in play at all times: do not exceed the max current, the max voltage, or the max temperature.

In summary: you've got really nice batteries. Somebody put out $$ for them. A quick google for "balance charger" turned up good options starting at $11.

Can you afford an $11 charger? Lithium fires are both extremely dangerous, extremely toxic, and extremely expensive.

I hope this helps!

I'm sorry I didn't ask a simpler question.

Thanks for your reply, dch. As a result of my experience (and that of others) over the past few years with, first, LiPoly cells, then LiFePO4 cells, I take plenty of care to balance LiPoly cells in packs (I rarely use them now, tho), but find it unnecessary for MY use of LiFePO4 cells, in packs as large as 20S4P. Please review your sources of information on overcharging.

The simple question is: How can the charging current into a battery pack be shut off when a particular voltage is reached? In my case, 60V and 75V (+/- 2V), and allowing for a currrent of up to 4A before shut off.

I have a commercial charger (elitepowersolutions.com) that can do that. It's designed for a pack of 16 20Ah LiFePO4 cells in series (the pack I use in my Big Dummy cargo e-bike; http://endless-sphere.com/forums/viewto ... =6&t=20858). It shuts off the current at ~58V. It runs on line current, 120 VAC. No way to utilize it's smarts in my PV system. I suppose I could use an inverter, and . . . but I'm looking for something simple.

I've been reading about solid state relays. Does anyone know if one of those might work, and where they might be obtained?

I'd really like to hear how you folks deal with the problem - preventing battery overcharge when charging with PV panels.

Thanks for any input!

Hi amobaroque,

No problem! I'm safety conscious, that's all.

60-75V * 4A, I'll assume you'll want to check out any relay pretty carefully before buying. So I'll skip that step.

The simplest voltage-based cutoff I can think of would involve an op amp, a zener diode (as the voltage reference), and some resistors. Total cost $1.50 or less. Before I start designing a schematic, what do you think of such a design?

I really can't say, but, before taking advantage of your very kind offer, I'd like to hear from a variety of people who have successfully dealt with this problem - in any way at all.

Hi Amobaroque,
I use a Plasmatronic PL20 but as you say, they aren't cheap, $299 on evil bay or sometimes a little cheaper second hand if You can find them. I have been readng up a bit on the rapid technoligy and capability improvements of the Li Po 4 cells on the various electric vehicle forums and they are quite impressive. Very temp stable and can handle some very fast charging to 80% with virtually no temp rise. How Long have you had your batteries?

T1 Terry