Charging Thundersky Lithium with Solar, How?

[VRdublove]

Hey guys,

So I'm beginning to formulate plans to build a tandem trike that will use a 48V, 60Ah Thundersky lithium pack. We plan on using two of SunPower's 315 watt panels. But the whole idea of charging lithium with solar power is throwing me for a loop. It's simple with lead acid, as it will really except whatever charge you throw at it, but what about the thundersky batteries?

Since Lithium prefers a CC/CV charge curve, how can one accomplish this constant current with intermittent solar that will be moving through shady and sunny areas? Will the lithium respond well to the interrupted current that it will receive from a standard solar charge controller? Will a standard PWM charge controller or a MPPT charge controller? Are there any solar charge controllers that are made specifically for lithium?

The only way I can see it working is by adding a small 12V lead acid battery that is charged by the charge controller to stabilize the intermittent current, and having another charger that runs off this 12V to charge the lithium, but that would add weight and complexity.

So can it be done simply with a good BMS on the lithium and a standard 48V charge controller? The curve is the same except for the Float phase at the end of the charging process.

Thanks-

 

[VRdublove]

Found the answer:

Genasun Lithium Solar charge controllers:

www.genasun.com


We will be using the Genasun GVB48-8 which can handle 48 volts and 8 amps of charging current.

 

[dak664]

Don't know about lithium preferring CC/CV, I would say limited-current maximum-voltage. Lithium isn't hurt by being below full charge for an extended period of time, in fact they last longer if the upper limit isn't pushed, and the recommended storage is at half charge. With lead acid you want to recharge as soon and fast as possible to reduce sulfation hence tracking, MPPT, or excess panels, but lithium is happy with any charge rate up to the maximum. If the panels maximum output is less than the recommended charging current all you need is a voltage limit, which the BMS may or may not perform. If the panel voltage is greater than the voltage limit a simple buck converter can do the voltage limit and will also trade off excess voltage for current. With say <C/5 of maximum panel current and a conservative voltage limit I'd leave it on solar charge all the time and transfer to a plug-in charger for fast top-up only when the current state of charge is not enough for your planned trip, or maybe once a month to ensure cell balancing.

 

[EVan]

Yes, you can do this.

You could simply use a cheap solar regulator set to the appropriate constant voltage, on the "gel lead acid cell" setting. This shorts the panels when the voltage rises too high, and allows them to charge the battery via a diode until then.

The HVC FET on your BMS will take care of the end of charge.

Variable current charging is not a problem for the cells. You will need an AH counting meter to know how charged they are though.

I'm not sure that you can get a MPPT regulator that will be economical for only 600W of panel but if you find one, report back

 

[jondoh]

Just wanted to make sure the practicalities of charging with solar are understood.

48v x 60 ah = 2880 wh

2880 wh / (315 watts x2) = 4.5 hours and that's if each panel is putting out over 24 volts

What this means is that you get to ride every 3rd day with a full pack-- unless you have a second pack to charge while the other is in use.

Solar is great but it's best for folks who can't get power any other way. Otherwise, if you plan to ride a lot, i'd rather spend the solar panel money on a good charger.

 

[EVan]

Umm, 4.5 hours comes in at less than one (good and sunny) day in my book

I agree with your point, but if it was all about the cost he should fit a 2-stroke engine to his trike!

 

[dak664]

Watts out can't exceed watts in, that's for sure (OK maybe by a miniscule amount with precise temperature control) but the OP wondered whether lithium could be solar charged the same way as lead-acid and that's a no-brainer: lithium happily accepts an intermittant charge while lead acid undergoes degradation until fully charged.