30 minutes of charging her batteries from the alternator does better than 8 hours of charging from her solar panels. I have gel batteries which due to their slower charge rate, I charge ONLY from my solar panels. But is there truth to what she said? I plan to spend a good bit of time in the far Northwest where rainy days are the norm, so I maybe need to change something (like switch to a kind of battery that charges faster, thus able to take advantage of the alternator?)
I'm late to the party and there has been much good input. But it's an interesting question (and specific use case) worth a bit more dissecting.
caveat: charging gel
I agree gel should not get the fast-and-furious alternator treatment, so the 16x faster issue doesn't apply here. For those considering gel for its
excellent cycle life specs, I'd limit charging to 0.2C (20A per 100Ah of capacity). This is most easily done from alternator via
DC-DC charger. 20A DC-DC are widely available and surprisingly cheap. In this situation we are talking about
augmenting solar, not blasting it out of the water with
16x moAr pOweR!!!!.<tm>
Renogy has a 20A for about $120 (~$100 from Amazon warehouses scratch-and-dent pile) and would be a nice addition for folks in the Northwest. I've done
some math on that topic here.
16x faster
Let's do some sanity checking on "30 minutes of charging her batteries from the alternator does better than 8 hours of charging from her solar panels" assertion.
There is only so much current a battery will accept, and that is dictated by rated capacity and battery chemistry. Let's use a deeply-cycled 100Ah bank and 200w of solar for our baseline since it's such a common size.
- Flooded lead acid will accept up to 0.2C (20A per 100Ah of capacity). If the alt were running for 0.5hours that means we are talking about 10Ah (20A x 0.5hrs). That would mean to if alternator charging is 16x the solar is only providing 1.25A, or something like 16w.
- AGM (of the sort most have here) will accept up to 0.3C (30A). So 15Ah harvest in a half-hour, equivalent to solar harvest of 1.9A (~24w).
- Lithium can accept 0.5C (50A). So 25Ah, or the solar 8-hr equivalent of 41w.
Think about your battery type/capacity and your own solar harvest observations in amps or watts to see how they stack up against a half-hour of alternator charging.
The 16x claim makes more sense when we talk about much bigger battery banks, like 300Ah+. Plain isolators will pass as much power as the batteries will accept (constrained by the alternator's ability). Scaling the solar (or DC-DC) for the bigger bank would cost considerably more.
Edited to add: based on YT thumbnails she camps in forests quite a bit. Shade can/will clobber solar harvest.
cooperation, not competition
It's not really either/or;
alternator charging and solar work spectacularly well together. Each addresses the shortcomings of the other and can decrease overall costs.
- isolator charging - cheap source of current, especially when bank is deeply discharged. But insufficient voltage (and probably time) to complete Absorption for lead chemistries
- solar - small installs are cheap sources of long-duration voltage. Insufficient current to meet lead batt charge current minimums, and may not have enough grunt on its own to get the bank fully charged in poor conditions. PWM-based systems in particular strugglen when bank voltage is lowest.
- DC-DC chargers - tend to deliver higher voltage but lower current than isolator for the money. IMO DC-DC is not always worth the money when solar is present.
As
@mpruet noticed, an optimal charging scenario would be
- isolator charging when batts are deeply discharged (roughly Bulk stage). This is likely in the morning when the voltage difference between bank and alternator is greatest. You could alternator charge later in the day during Float but it wouldn't do much.
- after the alternator blast solar charging takes care of the lower-current, higher-voltage, longer-duration Absorption and Float stages.
Full disclosure: I have
a voltage-sensing relay for alternator charging and
copious solar. Solar does 95% of the work although the alternator gets to contribute if I'm driving and solar conditions are poor. Even though it doesn't get much use I am 100% satisfied with the low cost, simplicity, and Big Current provided by the VSR.
Thank you for attending my Ted Talk.
