Re: [mowbot] Battery Exchange ?

robin nospam at
Tue, 24 Sep 1996 16:04:05 +0100

Monta Elkins <Monta nospam at> wrote:
> What's a good 'battery charge' to 'run' ratio?
> I'd guessed 2:1
The small 3Ah lead-acid battery I had been thinking was a prime candidate
needs a charge of 0.5A for 7h. The larger 6Ah version needs 0.5A for 13h.
Assuming Mowbot runs at about 1A, that is close to 2:1.

Much larger batteries can be charged faster, so perhaps we had better
think of using these: for instance, a 24Ah battery recommends a 2.6A
charge for 9h. Even if Mowbot is scaled up a bit and now needs 2A to run,
that brings the charge/run ratio to better than 1:1. The down side is
that this battery is much bigger and heavier and costs 60 pounds ($90).

> Given 2:1 we get 16 hours charging for 8 hours work.
> Seeing that time could be a problem; I was wondering
> if anyone had an ideas for a simple 'battery exchange'
> method.
I had considered it in connection with the charging nest idea, but unless
we are forced to deal with it, I would really rather avoid thinking
about how to make 12V, 1A terminals reliable, detachable and gunk-proof.
It's going to be hard enough to get the charging contacts reliable.

> If we could work out a simple battery exchange method
> we could work at least daylight hours if not 24.
> i.e. If mowbot can find 1 exchange station; it
> could easily find 1 of 2 battery exchange stations
> and switch back and forth. (Or one station that
> can manipulate 2 charging batteries).
Nice idea! But another point I made a while ago is the cost of solar
panels is high, so we might not have even one such station (much as
I would like it), let alone two. If the station is mains-powered,
it can certainly draw enough current to keep any number of batteries
charged up ready.

> 3 batteries, 1 in use, 2 on charge.
This reminds me of a TV programme I saw a long time ago which showed a
battery-powered fork-lift truck using its own motive power and hoist to
change its own battery. Good engineering.

> [I'd thought there may need to be a second battery
> on mowbot used only during the exchange cycle
> that charges during the mow cycle from the main battery,
> with the battery exchange functions designed to operate a 9v]
Depending on how fragile the power supply turns out to be, a tiny
5V backup battery for the electronics might be a wise investment.
Carrying two 12V batteries around seems a bit excessive. Though it
might have mechanical advantages for weight distribution.


R.M.O'Leary <robin nospam at> +44 973 310035  P.O. Box 20, Swansea SA2 8YB, U.K.