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I'd like to go solar with a couple of our battery-powered tools without a) charging the batteries with DC directly, as they're expensive batteries with their own fancy AC chargers, or b) spending money on other expensive solar batteries to power charging these expensive tool batteries.

More specifically, I have a tool shed I'd like to put solar panels on solely to charge the electric mower & weed whacker (two 7.5ah@56v/420wh EGO brand batteries), and a bike shed I'd like to do the same - charge three Bosch 500wh (36v) battery packs with solar on the bike shed. We drain all batteries to about half capacity once a week, so they have roughly similar requirements - generate about 1kwh per week, although more than that is better "just in case."

I can get used 250w, 30v panels locally for about $90, so I figure four of those should be enough per shed, but I'm new to solar. I live in Seattle, so I figure 1kw of solar capacity, when factoring in clouds, conversion losses, and other system losses, should be enough to get 1kw into the batteries over the course of a week in most conditions, but I could use a sanity check there. Since the panels are relatively cheap I figure it's a good place to spend money as opposed to solar battery capacity, which seems very expensive.

Speaking of batteries, that's my question. There's lots of advice on how to hook up a solar array to a big battery to run your fridge at night etc, but not so much for my use case.

Ideally I'd just charge the tool batteries straight off an inverter, without a solar battery, or use a small (and inexpensive) battery. But I have the intuition that I can't just do that with typical equipment - it's here where I start to get fuzzy... both the EGO tool charger and Bosch battery charger draw about 2a while charging (one a little less, one a little more). So take the Bosh charger, 120v @ 2.2a = 264w, and the internet says an inverter draws 1 amp to output 20w at 24v, so that would mean I'd need a 264/20=13.2amp battery? The cheapest decent-looking LIFEPO4 I can find would be four 12v 7ah Dakota batteries (two pairs in parallel would give 24v 14ah), which would run about $200, and it seems like that's just scraping by for the requirements.

  1. Would this work as I've imagined it? Surely I'm missing something.
  2. Any way to cut out the charge controller & battery middleman? I'm aware of microinverters, would it be better to use microinverters and a voltage regulator to get 120v without going through a charge controller?

Thanks for any input!

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  • Hi Woahdae, thanks for the great question and welcome to Sustainability on Stack Exchange. Are you most interested in optimizing your costs, minimizing your environmental impact, or expanding your off-grid abilities? Your priorities might affect the answer. :)
    – Nic
    Sep 2 at 6:16
  • Is it safe to assume that you don't have mains power at the sheds, so this is essentially an off-grid system?
    – LShaver
    Sep 2 at 14:41
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    I have a long extension cord currently but yes I’d like it to be off grid.
    – Woahdae
    Sep 2 at 14:59
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    @Nic, I'd say my order of priorities are expanding off-grid abilities (not that I need a weed whacker in an emergency, but it'd be nice to cut out the 200ft extension cord), optimizing costs, then minimizing environmental impact, but all are motivations. The tools cost about $600 so it'd be nice not to spend more than that on the solar charge system...
    – Woahdae
    Sep 2 at 15:23
  • From my limited experience with 18V tools and a 36V bicycle the answer is: AC and a small 12V battery. Get a charge controller that you can set the cutoff etc, and keep those conservative (cycle the battery between 60% and 80% SoC, it's not there to provide actual charging, just to buffer the SCC). Finding random-voltage MPPT controllers with decent LiPo battery charging profiles is hard. Better to use the AC charger they came with.
    – Móż
    Oct 5 at 6:16
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After more research, I think a 12v 200w system is more calibrated to my use case. Since my load will be 120v, 2.2a, that'll draw not quite 300w out of an inverter, so at 12v that's 25a. If the panels produce 20a, that might do it, or maybe it'll need an extra ~5a from the battery. I found a 12v 16ah battery for $60, which should satisfy the use case of charging the tool battery mostly from the sun with a small solar buffer battery. It's beyond me how to remove the charge controller and battery from this system, but that's only $110 of equipment. Good enough!

I think there's a policy here against linking to products, so I'll be generic:

$500 planned total:

  • 2x 100w 12v panels, $100ea
  • Cheap 500w 30a 12v inverter, $40
  • 20a PWM charge controller, $50
  • 12v 16ah Lifepo4 battery, $60
  • 2x RV tilt mounting brackets, $35ea
  • Misc wiring, $45
  • Low voltage disconnect (since battery can't handle full load w/o solar), $25

I'll try to remember to post back here and accept this answer after some testing, but in case I forget, here's a breadcrumb for someone else.

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    You can link the product as long as you clarify whether you are affiliated with the manufacturer/vendor or not. Asking for specific product recommendations is what's frowned upon.
    – LShaver
    Sep 3 at 0:41
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    There are also combined inverter/charge controller systems designed for RVs and boats, which could be cheaper and also have less components (so less failure modes and less conversion losses). Here's one example
    – LShaver
    Sep 3 at 0:55
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Here's a potential answer one of my friends gave me. I'm still leaning towards a more traditional solution but:

This video shows the internals of the particular charger I have for the EGO system, and it appears that after the transformer, it uses about 60v DC, so my friend is suggesting I verify this with a multimeter and if accurate, wire up leads after the transformer and power it with two 30v solar panels in series.

With 250w panels, that would put 500w/60v=8a into the charger, which normally has a load of 210w @ 120v (1.75a). Would the charger just ignore this extra juice? Fry the charger? Put too much into the batteries?

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    You'd be bypassing the control mechanisms this way which could damage the batteries and/or start fires, unless you had a thorough understanding of how the EGO system worked. The fact that there is a data line from the charger to the battery makes me think you'd have a very hard time reverse engineering the system.
    – LShaver
    Sep 3 at 0:36

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