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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 7.5 Ah @ 56 V/420 Wh EGO brand batteries), and a bike shed I'd like to do the same - charge three Bosch 500 Wh (36 V) 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 250 W, 30 V 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 1 kW 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, 120 V @ 2.2 A = 264 A, and the internet says an inverter draws 1 amp to output 20 W at 24 V, so that would mean I'd need a 264/20 = 13.2 amp battery? The cheapest decent-looking LIFEPO4 I can find would be four 12 V 7 Ah Dakota batteries (two pairs in parallel would give 24 V 14 Ah), 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?
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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, 2021 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, 2021 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, 2021 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, 2021 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, 2021 at 6:16

3 Answers 3

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Charging directly with solar panels isn't advisable. The problem is that as clouds obscure the sun or birds fly over your solar panel, power production fluctuates. It would fluctuate so much that nearly every charger would get confused.

So instead use solar to charge a floated 24V lead-acid battery system. It doesn't have to be large, just make sure it can handle the charging current. For example most sealed lead-acid batteries can be charged with 0.3C, so if you want 36 volts * 4 amperes = 144 watts (let's make it 180 watts due to inefficiencies) for ebike battery charging, just add a lead-acid battery that can be charged with 180 watts. At 24 volts, that's 7.5 amperes which would mean 25 ampere-hours of battery capacity is enough (according to the 0.3C rule). For example for 24 volts, you need two 12V 28Ah batteries that cost around $120 both together.

Then add a modified square wave inverter that's at least 300 watts to the system. You should find one for less than $50 at least for 24V systems.

Then you just wire solar panels with a lead-acid charge controller (preferably MPPT but if you really want to cheap out you can get PWM) to your system. With MPPT, the panel nominal voltage needs to exceed lead-acid battery voltage, with PWM the nominal voltages need to be the same or else your power production suffers.

A 24V 300W inverter has about 6 watts of idle power use. Compared to 144 watts of charging an e-bike battery, that's nothing.

If you want to charge very small batteries like Sanyo/Panasonic Eneloop 1.2V 2Ah AA batteries or your phone or tablet, then you have several options:

  • Get a 12V/24V system powered USB power output, you can get one for as cheaply as below $10 for both cigarette lighter sockets and for permanent wiring. Then you can use USB AA/AAA chargers like GP ReCyko M451. You can also charge any USB device like your phone or tablet. With USB power outputs that support power delivery, you may be able to charge even a laptop computer!
  • Get a 12V/24V multi-voltage output DC-DC power converter that supports 12V output voltage (and probably has a lot of other choosable output voltages). Then you can use chargers like Maha/Powerex MH-C9000 Pro that use 12 volts, 2 amperes to charge up to 4 AA/AAA batteries.

The reason I don't recommend using inverter to charge AA/AAA batteries but do recommend using inverter to charge larger batteries like ebike and tool batteries is that small AA/AAA batteries are charged so slowly that the inverter idle power usage would waste lion's share of the energy. However, large batteries like ebike and tool batteries are charged with so much current that direct DC/DC converter would probably be so expensive that it just makes sense to use a modified square wave inverter + the original mains-powered battery charger you already have.

Don't choose a sine wave inverter. They have way more idle power loss.

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After more research, I think a 12 V 200 W system is more calibrated to my use case. Since my load will be 120 V, 2.2 A, that'll draw not quite 300w out of an inverter, so at 12v that's 25 A. If the panels produce 20a, that might do it, or maybe it'll need an extra ~5 A from the battery. I found a 12 V 16 Ah 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, 2021 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, 2021 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, 2021 at 0:36

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