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solar charging up a 48 VDC lead acid lawnmower pack

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Posted by danny burstein on July 20, 2014, 3:16 am
I picked up a used 48VDC Sears lawnmower a couple of years ago.

It uses 4 12V batteries in series, each one a nominal 9 amp hour
rating, so (at nameplate maximum) I'd have 48 times 9 => 450 watt-hrs.

Reality, of course, is a bit less and I have my own back/movement
issues, but I get a reliable 15 minutes (and then some...) out of it.

It uses a 120VAC charger which plugs in, takes about 65 watts
from the outlet (measured with a KAW) for an hour or three, then
down to 25 or so, then trickle charges at 2.

(I don't have an easy way to measure the DC output that
goes into the assembly).

I'd like to hook up a solar panel and charge it that way.

So my key question is... anyone have a recommended charge
controller to pump in at the higher rates, and then
shift downward as the batteries get full?  

I'd really rather not waste the expense and energy in
doing a (nominal) 50 volt-from-the-array conversion ->120VAC
and then the regular charge cord...

Thanks for all suggestions.

Knowledge may be power, but communications is the key
[to foil spammers, my address has been double rot-13 encoded]

Posted by mike on July 20, 2014, 4:44 am
On 7/19/2014 8:16 PM, danny burstein wrote:

sorry, but I have more questions than answers.
Most importantly, why bother???
Costs you a nickel to charge it.
say 26 charges/year, that's $.30/year in electricity.
How much can you build for $.30?

Why do you care about efficiency?  What are you gonna do with
the energy that you didn't waste?  Consider tapping that source.

Ok, OK, so everybody has to have a hobby.
The biggest problem with solar is that it varies a LOT.
Most circuit designs you'll find depend on some relatively
stable input.  They will go all crazy when the input fluctuates
below what they want.
Stabilizing the input is harder than regulating the output.

You're gonna need a custom design.

IMHO, you'd save far more energy by building some solar storage
facility that converts to 120VAC and using that for your infrequent
mower charge.  Then you could actually use all the rest of the
energy collected for other purposes.
Solar panels are expensive.  Use all you can get out of them.

Posted by danny burstein on July 20, 2014, 5:08 am

The area being mowed is pretty far from the building and  
any outlet. Moving it back and forth is a hefty pain.

I'd like to just have a cheap shed for weather protection
around it and use solar panels.

If I figure on needing 200 watt-hours for a charge (full
nameplate is 450...), then 75 watts of solar panels
should be adequate if there isn't too much loss.

(rule of thumb: solar cell output times four gives  
you the nominal daily output).

The panels should, depending on sunlight and
the battery state, give me very roughly 60 volts
of DC output. (four or five 12 Volt panels in series)

At five dollars/watt, 75 watts worth would be $00.
Throw in another hundred for an MPPT controller, and
we're at very roughly $00.

Much cheaper than running a power cable the 500 or so feet.

Knowledge may be power, but communications is the key
[to foil spammers, my address has been double rot-13 encoded]

Posted by mike on July 20, 2014, 7:44 am
 On 7/19/2014 10:08 PM, danny burstein wrote:

I did a similar installation design for a mountain-top repeater.
Also did sun tracking, but you don't need that.

Basically, you have to examine your local insolation data.
At the worst time of year, you only need to accumulate
200 Whr of energy between mowings.  That tells you how big
your solar panels need to be.  I'm guessing that it's  a lot
less than 75W.
Start with 12 or 24V panels.  Use a microcontroller
to build a boost converter to get up to the 56V you need.
Dither the duty cycle keep the charge current at maximum,
that's the mppt part.  Stop the charge when the target voltage
is reached.  Since it's only code, it's not hard to step the
voltage down if you want.

There are losses everywhere in the process, so you'll need
to scale it up to cover those losses.

The advantage of this technique is that it will provide charge
under a very wide range of light levels.

I have no idea the details of your situation, so I'll make up
some numbers.
If you mow every 7 days, you need 29Wh/day and using your
4x number, that's 7.5W of panel. YMMV...Your Mowage May Vary.
Multiply by some number for losses, bad weather etc.

Or, you could just series  6 12V 2W panels and use a big zener
diode to clamp the voltage at 13.6V/battery and be done with it.

All the fancy stuff is great for maximizing use of solar.
You don't seem to need any of that.

Posted by Jim Wilkins on July 20, 2014, 11:18 am
For lowest cost I'd bring out each battery on an Anderson connector  
and charge them separately and sequentially, perhaps one per day, with  
a 12V charger. This will charge each battery to its full capacity  
though you can safely use only the total Amp-Hours of the weakest.

The DC equivalent of a Kill-A-Watt:
(Amazon.com product link shortened)
Mine is accurate to no better than 0.2A despite the 0.01A display  
resolution so its integration of Amp- and Watt-hours while  
trickle-charging doesn't mean much. It may be more useful at higher  
discharge currents.

It reads voltage in either direction, but (+) current only from Source  
to Load, you'd have to reverse it to display discharge current.

Leaving a sealed battery on the solar charger for a long time may  
damage it:


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