questions about building a solar generator to power electric lawn mower

Baxter

I'm new to solar energy and trying to learn as much as I can before I invest any time and money. My goal is to start a solar lawn care business in Alabama, and have a solar panel attached to the top of a truck tool box, with battery and inverter inside so I can charge my batteries in between mowings. I know it is possible because there are other business doing it out there. I have a 36 volt lawn mower and a 18 volt trimmer that has about 4 batteries.

The charger for the lawn mower plugs into a regular outlet and reads Input: AC 100-240v 47-63Hz 0.3A Output:36V - 0.7A The rechargeable battery just has 36v 10ah on it. My question on that is, what size panel, inverter, charge controller, and battery would I need to keep it charged or topped off in-between cuts? I was thinking a 120 watt panel, xantrex true sine wave inverter SW600 540 Watt, 30 A charge controller, 12 volt 100ah AGM battery...Am I close to being right on this?

Everything will be located inside the truck tool box to keep the elements off of it, but I just not sure if I'm going in the right direction on this. Any advice or opinion would be beneficial.

Cariboocoot

Re: questions about building a solar generator to power electric lawn mower

Welcome to the forum.

Yes, it is possible. From a technical point of view it's about as practical as a chocolate hammer. Grid power is cheaper. But that doesn't include any marketing benefits from the 'green perception'.

What's missing from your numbers is time: how long does it take to recharge the mower and the trimmer? If you drain them down completely, then plug them in through a Kill-A-Watt you will get a fairly accurate number for the maximum Watt hours required. From that you can figure out what step to take next.

Direct DC charging: this is more efficient, but can be unreasonably complex to instigate. For example your battery @ 36 Volts is "non standard" in the RE world; only a few controllers will operate at that Voltage and they are not cheap. They also would be overkill on capacity and features for what you want to do. Otherwise you're looking at 1 Amp @ 36 Volts (probably 42 for charging) which is <50 Watts. Easy, right? Except that panels aren't 36 Volts either. Oh, some are; but they tend to be fairly large in Watts. So you'd be connecting a few small (around 20 Watts) 12 Volt panels in series. Naturally the small panels cost more per Watt than the big ones. The trimmer at 18 Volts is slightly above normal for a 12 Volt system and no charge controllers off-the-rack. Bummer.

AC based charging: less efficient, but easier to do. Figure out the power needed, small inverter runs off small battery recharged by small panel. By small I mean:
The ~36 Watts for the mower over 12 hours (say) is probably 300-360 Watt hours (it's not linear charging). On 12 Volts that's around 10 Amp hours needed, so a minimum 20 Amp hour battery. Pretty small, eh? And all you'd need for that is about a 40 Watt panel and a 4.5 Amp charge controller. The trimmer's demands will of course increase this. But first you have to find out the real need.

It is somewhat ironic that the panel on the truck will be charging a standard battery which charges the mower & trimmer later, as it means no direct transfer from solar to the batteries actually doing the work. Yes, this cuts down on the efficiency (100% of power stored in and taken from the other battery). But few people would want to see the truck sitting in the sun charging up until you have enough power to mow their lawn - at night.

See what I mean about practicality? Plugging in overnight just makes more sense. Although it may not sell as well.

BTW these numbers I've given are just rough estimates; without an actual power usage number you can't be more precise.

Baxter

Re: questions about building a solar generator to power electric lawn mower

Cariboocoot wrote: »

Direct DC charging: this is more efficient, but can be unreasonably complex to instigate. For example your battery @ 36 Volts is "non standard" in the RE world; only a few controllers will operate at that Voltage and they are not cheap. They also would be overkill on capacity and features for what you want to do. Otherwise you're looking at 1 Amp @ 36 Volts (probably 42 for charging) which is <50 Watts. Easy, right? Except that panels aren't 36 Volts either. Oh, some are; but they tend to be fairly large in Watts. So you'd be connecting a few small (around 20 Watts) 12 Volt panels in series. Naturally the small panels cost more per Watt than the big ones. The trimmer at 18 Volts is slightly above normal for a 12 Volt system and no charge controllers off-the-rack. Bummer.

Thank you for all your information. The charger plugs into 110 volt normal plug, so how do I just emulate that for 8 hours? This company has managed to make it work for them, and they have just one panel mounted to the top of their roof. http://cleanairlawncare.com/ check them out. So I really just need to be able to top my mower battery off during the day, because I won't be draining it all the way, and the mower has the ability to take the battery out, so I could be mowing and charging a spare battery at the same time. I won't be charging 36 volts directly, it is just a 36 volt battery that powers the mower...I think the charger is what I need to focus on. Right?

mike95490

Re: questions about building a solar generator to power electric lawn mower

I think it's much more likely they have a PV panel bolted to the roof, and wired to the 12V system. Do they stop mowing on cloudy days ?

Rely on the vehicle alternator to do the majority of the of the vehicle battery recharge. That powers a 120V inverter with a half dozen chargers and batteries in the tool box. That's the way I'd do it. To set up a large enough PV system, that would be 100% independent of the vehicle, and only rely on a 200W PV panel, would be overkill, but if they got a good marketing department, maybe they do it.

Cariboocoot

Re: questions about building a solar generator to power electric lawn mower

Right.

You need the AC system, if for no other reason than it is much easier to do.

The Kill-A-Watt plugs in to an outlet. You can then plug your chargers in to it and it will measure how much energy is needed to recharge the mower and trimmer. Then you will know how much power you need to supply.

This will come from the battery through the inverter. Neither of these are big power users in terms of Watts (power at the moment) so a large inverter is not needed. Probably <100 Watts in fact. But over time they may use a large amount of electricity; Watt hours. So a sizable battery may be in order, albeit probably not 100 Amp hours.

The panel & charge controller will replenish that battery and need to be sized accordingly. For example your initial choices don't line up: 100 Amp hour battery wants 10 Amps peak charging current which a 120 Watt panel will not supply and that doesn't need a 30 Amp controller. So let's look at three examples designed around those three components:

100 Amp hour 12 Volt battery. To recharge you'd pick 175 Watts of panel and a controller capable of at least 10 Amps. There aren't any 175 Watt 12 Volt panels though, so it gets messy and expensive.

120 Watt panel can provide 6.8 Amps and so needs a controller of at least that size. 10 Amps would be the nearest that would fit. This would be 'full charging' for a 68 Amp hour battery; on the low end for 100 Amp hours.

A 30 Amp controller could provide enough current for 300 Amp hours of battery and would need over 500 Watts of panel to do so. That obviously is not what you have in mind.

One limitation you have is the same with any mobile application: space. Figure out how much room you have for the panel, because you won't be able to fit more than that. That will dictate how much charging current you can get, and thus how big a charge controller you need and battery you can handle. Monocrystaline panels are more efficient for the square area, but not always available in every size and tend to cost more per Watt. A 130 Watt '12 Volt' panel capable of 7 Amps is about 58" long and 26" wide. Cost $285 http://www.solar-electric.com/solartech-spm130p-s-n-130-watt-multicrystalline-solar-module.html/ Basic charge controller $48 http://www.solar-electric.com/inverters-controllers-accessories/chco/mochco/stpwmchco/ss-10.html Small 75 Amp hour 12 Volt battery $156 http://www.solar-electric.com/batteries-meters-accessories/batteries/unba/unba75amagms.html Inexpensive 150 Watt inverter $113 http://www.solar-electric.com/inverters-controllers-accessories/inverters/sasiwain1/samlex-pst-series-pure-sine-wave-inverters/sa150wa12vos.html

That's just one option.

I don't suppose this mowing equipment has interchangeable batteries? You could have one on charge while using another. That would be more efficient.

Baxter

Re: questions about building a solar generator to power electric lawn mower

mike95490 wrote: »

I think it's much more likely they have a PV panel bolted to the roof, and wired to the 12V system. Do they stop mowing on cloudy days ?

Rely on the vehicle alternator to do the majority of the of the vehicle battery recharge. That powers a 120V inverter with a half dozen chargers and batteries in the tool box. That's the way I'd do it. To set up a large enough PV system, that would be 100% independent of the vehicle, and only rely on a 200W PV panel, would be overkill, but if they got a good marketing department, maybe they do it.

Interesting concept...where could I find out information on how to use my vehicles alternator to supply power to 120v inverter?

Cariboocoot

Re: questions about building a solar generator to power electric lawn mower

Baxter wrote: »

Interesting concept...where could I find out information on how to use my vehicles alternator to supply power to 120v inverter?

That's easy: connect inverter to vehicle's battery.
As long as the battery is large enough to supply the power needed while stationary and the alternator is large enough to recharge same it works. The solar panel on the roof will add a minimum amount of charging while stationary, but a maximum amount of (somewhat misleading) advertising value.

Now if you want to really 'out green' them you need to buy an electric vehicle to haul your electric mower with.

Baxter

Re: questions about building a solar generator to power electric lawn mower

Cariboocoot wrote: »

.

One limitation you have is the same with any mobile application: space. Figure out how much room you have for the panel, because you won't be able to fit more than that. That will dictate how much charging current you can get, and thus how big a charge controller you need and battery you can handle. Monocrystaline panels are more efficient for the square area, but not always available in every size and tend to cost more per Watt. A 130 Watt '12 Volt' panel capable of 7 Amps is about 58" long and 26" wide. Cost $285 http://www.solar-electric.com/solartech-spm130p-s-n-130-watt-multicrystalline-solar-module.html/ Basic charge controller $48 http://www.solar-electric.com/inverters-controllers-accessories/chco/mochco/stpwmchco/ss-10.html Small 75 Amp hour 12 Volt battery $156 http://www.solar-electric.com/batteries-meters-accessories/batteries/unba/unba75amagms.html Inexpensive 150 Watt inverter $113 http://www.solar-electric.com/inverters-controllers-accessories/inverters/sasiwain1/samlex-pst-series-pure-sine-wave-inverters/sa150wa12vos.html

That's just one option.

I don't suppose this mowing equipment has interchangeable batteries? You could have one on charge while using another. That would be more efficient.

So in theory, would the system mentioned above work for my application (on a perfectly sunny day)?

Yes the 36 volt batteries are interchangeable, so I plan on purchasing a few more.

Baxter

Re: questions about building a solar generator to power electric lawn mower

Cariboocoot wrote: »

That's easy: connect inverter to vehicle's battery.
As long as the battery is large enough to supply the power needed while stationary and the alternator is large enough to recharge same it works. The solar panel on the roof will add a minimum amount of charging while stationary, but a maximum amount of (somewhat misleading) advertising value.

Now if you want to really 'out green' them you need to buy an electric vehicle to haul your electric mower with.

I'm not trying to mislead anyone, and if it won't work efficiently, then I won't bother spending the money...I just had the idea a few years ago and of course looked it up on the internet and someone else already thought of it. I don't care how it gets powered, solar, alternator, wind, I just don't want to use gas. When gas skyrockets again I'll hopefully have an alternative energy source for charging my lawn equipment.

Now if there are solar generators on the market that can power TVs and refrigerators, then I don't understand why my idea seems so difficult to achieve?

Actually I have a V6 Tacoma, so maybe I'll be balancing out some carbon...thats funny electric vehicle. You're too funny!

ZoNiE

Re: questions about building a solar generator to power electric lawn mower

Like Cariboocoot said, it is easy, connect a *GOOD* pure sine wave inverter directly to the vehicle battery, but Go one better, add a changeover relay which switches your inverter to run on the car battery when the engine is running (you are driving between clients) and off the charge controller and another solar charged battery bank when you are parked. This way you cannot strand yourself if you are charging batteries while parked if you happen to be mowing that really really big lawn. You could also add extra batteries to the vehicle and forego the relay, but you could potentially strand yourself.

An inverter putting out 10A of current @120 VAC to a battery charger will draw 100A from your car battery. You can drain your vehicle battery in a couple of hours and then the alternator will be screaming like a pig when you start (luckily) the engine and it charges that dead battery...

You may be able to charge the 36V batteries directly from the solar array with a Midnight Kidd charge controller, but not sure how those tool batteries will respond since they usually have electronics (or temp sensors) inside that talk to the charger that came with them.

if it were me, nothing better to power a lawnmower with than good old gasoline, But I get what you are doing, and it is a good idea.

Cariboocoot

Re: questions about building a solar generator to power electric lawn mower

Baxter wrote: »

I'm not trying to mislead anyone, and if it won't work efficiently, then I won't bother spending the money...I just had the idea a few years ago and of course looked it up on the internet and someone else already thought of it. I don't care how it gets powered, solar, alternator, wind, I just don't want to use gas. When gas skyrockets again I'll hopefully have an alternative energy source for charging my lawn equipment.

Now if there are solar generators on the market that can power TVs and refrigerators, then I don't understand why my idea seems so difficult to achieve?

Actually I have a V6 Tacoma, so maybe I'll be balancing out some carbon...thats funny electric vehicle. You're too funny!

Ah. The infamous "solar generators". The term itself is apocryphal.
Of course it can be done. One of key factors is "how much power for how long?"
Refrigerator, for example. We've seen companies claim their "solar generator" can run a refrigerator. The battery size inside it is too small to run the average unit for a day, but they don't ever mention for how long it will run it. Misleading, no?

But yes what you want to do can be done. Fairly easily as I outlined above. The most important thing if you want to go totally solar for it is to measure the power requirement and then design a system that can supply that. Otherwise you are guessing and have a 2/3 chance of getting it wrong: either too big or too small.

Or you can borrow power from your Tacoma's alternator. Most of the time when you're driving around it isn't doing much. But on the other hand using a vehicle engine as a generator is pretty far from efficient - and far from green.

Putting in a 'full green' system is perhaps not practical, especially as Mike mentioned that when the sun doesn't shine it produces nothing. I run this place here off solar. So do many others. It takes a lot of battery and panel to do so, and most of us end up running generators when the sun doesn't co-operate. It may not be totally green, but it's better than burning fuel all the time to have electric.

westbranch

Re: questions about building a solar generator to power electric lawn mower

Linking thread topics (there is a thread on LiFePO4 batteries) the LiFePO4 batteries can take less than a full charge without issue. http://forum.solar-electric.com/showthread.php?23898-GBS-LiFePo4-20-and-40ah-batts&p=195650#post195650

You might want to look into the battery chemistry before you decide on which brand to buy, as some of the older battery chemistrys have recharge issues, like a hot ( discharged ) battery will not recharge until cool enough....

Baxter

Re: questions about building a solar generator to power electric lawn mower

I appreciate everyones input about this topic. I have always been interested in solar energy, and based on the responses it sounds like it isn't as feasible of an idea as I thought. I'll have to do some more research and figure out how long it takes to charge and work from there. Thanks again!

PNjunction

Re: questions about building a solar generator to power electric lawn mower

Hmm... I'd love to see the ACTUAL charging circuitry these mowing companies use.

However, if all I wanted to do was "top off" and advertise, what I'd do is pop a panel on top of the truck, ANGLE it when I arrived not leaving it flat for more visibility, and have a simple AGM in the tool compartment with a charge controller and a small ac inverter.

Then, for "topping off", the little wall-wart overnight charger would plug into the inverter putting just a little bit back into the mower batteries while in the field. That would make the charging led's glow for visitors or prospective customers.

I'm not saying I'm totally recharging the battery, and never mention if I allow these mowers to fully charge from the household AC when I get home overnight to start the next day fully charged. In this case, solar sustainability has a LOT of leeway in the definition.

The devil's in the details!

BB.

Re: questions about building a solar generator to power electric lawn mower

So--To summarize:

1. Get a Kill-a-Watt type meter to measure the charging loads of your tools (peak watts, Watt*Hours per day/10 hour shift)
2. Add up the Peak Watts (size the AC inverter) and Watt*Hours per 12 hour shift (size the battery bank). You may need to model your time working vs driving (i.e., 1 hour working, 15 minutes driving), or simply say (worst case) battery supplies all power for 10 hour shift.
3. Design power system like an RV. Use a "battery isolator" to connect truck 12 VDC to your tool charging bank (more discussion needed). Use truck AC system to charge battery bank when driving.
4. You can now select all the components to install your system and have enough power to make it through a 10 hour shift--Get home, and plug in an AC battery charger to fully recharge the AUX battery and the tools for next day's use.
5. Now, if desired, choose Solar Panel and Solar Battery Charger to connect to AUX battery bank. Mount solar panel(s) flat to roof.

It is possible, if you park in sunny areas and have large enough solar panels, you can skip the AC battery charging at night--But I would guess AC charging is still needed--Bad weather, winter weather (wet/heavy grass to mow, etc.) is still going to need energy. Solar panels simply do not work in shade/dark cloudy weather.

More or less, the connection to the truck alternator is to keep your AUX battery happy during the day. Cost efficiency wise--Design a larger AUX battery can recharge from AC / Solar panel and avoid the Truck Alternator... The alternator will cost more in fuel than AC utility power will cost you. However, would you even notice (SWAG) $2 worth of extra truck fuel per day? Would it be worth the work to install a battery isolator/truck wiring for truck DC power during the day (having an extra source of backup DC power, probably worth it--Come 2pm and 5 more customers--And dead batteries in your tools--Not a good idea).

-Bill