My partner and I decided to keep our separate houses when we committed to each other last year. Her house is a beautiful timber-framed straw bale design that is located over a mile from the road on a 800 vertical foot driveway. It’s located on a 600 acre land trust and I’ve managed to create some pretty awesome single-track trails that run down the woods to the bottom. Her power system is a 1.5 Kw solar system with 4 6v Lead Acid golf cart batteries that hold about 232 amp hours in each battery. On sunny days it will usually charge up the batteries to max capacity in about 2-3 hours.
The problems arise when you try to charge a 20 Ah Lithium cell on the system. The charging cycle for my battery is about 8 hours for a full charge, and if I don’t time it right it can put quite a load on the batteries. How can a 20 Ah battery drain down a 232 Ah battery? It becomes a question so efficiency of charging and converting AC to DC.
Thilde’s system is basically a DC system, in order to charge by DC battery I have to use her AC inverter to convert to AC before it turns right back around and converts it back to DC. Every time you go from DC to AC or back again you have some power losses. It actually takes energy to convert back and forth. Why not go straight from DC to DC? That would make more sense right? The problem is the lack of availability for Cheap DC/DC LifePo4 chargers. The ones I use are about $35 shipped from China. I’ve never even seen a DC LifePo4 charger, but if someone made one, it would be a lot more than $35 because it would essentially be a specialty item.
Lithium batteries need the correct amount of voltage at the correct time. Over charge them and they can ignite or explode. Although LifePo4 is far more stable (and heavy & expensive) than Lipo, you can still easily trash your battery from mismanagement.
The 232Ah label on the lead acid batteries seems more than it really is because it is only a 6V system. Those Lead acid batteries would actually be only 29Ah if they were 48v. This means that a giant 70 lb Lead acid has only nominally more energy carrying capacity as my 16 lb LifePo4 battery. It makes more sense to think of it in Watt Hours not Amp Hours. Watt hours is equal to the Voltage or a battery times the Amp Hours of battery.
20Ah 48v LifePo4 is equal to 960 Wh (20 x 48 = 960)
232 Ah 6v Lead acid is equal to 1396 Wh (232 x 6 = 1396)
Total energy storing capacity of the solar system is 5584 Wh
Now we have to consider that we can only drain the Lead Acid system down to 50% if we don’t want to risk damaging the batteries. Now we are looking at 2792 Wh available to power our whole house and charge your bike batteries so you can recreate.
You start to see the problem. In the wintertime it is often so overcast, sometimes for days that you have to run the generator. The most that the generator can dump into our batteries with our bulk Lead Acid charger is 840 Watts. On cloudy days we generally run the generator 15 minutes in the evening and/or 15 minutes in the morning. I can run the trails in her woods about 4 times in 6-8 inches of powder from top to bottom and then back up the driveway before my battery goes dead.
So how do you deal with all these limitations? The best way to deal with charging an ebike battery on an offgrid system is to change your riding habits. In the same way that we only vacuum the house when it is sunny (the shop vac can pull 1400 watts) I only charge the battery late morning when there is some sun. That way it can go through its whole 8 hours cycle uninterrupted. I usually go out first thing in the morning and ride before Thilde wakes up then when I am done I throw the battery on the charger if it is going to be sunny. If the battery is fully charged I’ll take another ride or two in the evening as well. Lipo batteries don’t really like to sit fully discharged or fully charged either. The best way to store them is with a medium charge, not a full charge.