Re: Charging Battery with Harbor Freight 350W

Rye Gewalt (
Thu, 11 Feb 1999 06:35:22 -0500

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West Wight Potter Website at URL
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William Longyard wrote:

> Rye,
> I'm interested in how long it would take for this generator to fully
> recharge a group 24 deep cycle battery. Any ideas?
> Bill


I spoke with the senior staff scientist here at National Scientific
Laboratories (I've been talking to myself again) and received the
following reply.

Batteries are rated in Amp Hours. Amp hours are sometimes considered to
be the number of amps that a fully charged battery will deliver over a
one hour period and be totally discharged at the end. Actually, the
marketing types have changed this so that it is generally the number
of amps the battery will deliver over a longer period multiplied by the
period in hours. This is because the number will be higher when the
battery is discharged in a slower mode --- and more representative of
actual real world operation. (See the FAQ below for a much more lucid
explanation of Amp Hours and Reserve Capacity)

Since charging is not 100% efficient you have to put more amps into the
battery that you expect to get out of it. Assume that the charging
efficiency is 50% (it's probably actually around 75%, but for this
example the numbers are easier to follow). For a 100 amp hour battery
(typical of a type 24) you would have to charge it at 1 amp for 200
hours, or at 2 amps for 100 hours ..... or at 15 amps (Harbor Freight
max. capacity) for 13.3 hours. Thirteen hours is overnight except when
you are using a noisy generator --- then it's all night. Charge rates
of higher that 15 amps require sophisticated equipment or a strong faith
in God. This is a rule of thumb and there are all kinds of cute things
that will mess up these numbers, but it's probably a good
approximation. Also remember the safety issues with acid, hydrogen and
dropping the dam thing on your foot.

Going through such calculations makes one really appreciate what comes
out of the wall at pennies per Kilowatt Hour. It also means square
YARDS of solar panel to recharge a dead battery in any sort of
reasonable period.

The virtual ( virtually nonexistent) research department here at
NSL recommends the following reference material:


This FAQ should probably be included as a link on some of the Potter
pages as it is full of really useful information.



I hope this helps.

Now I will go back to Lurking ---


Rye Gewalt
'90 HMS18
Springfield, VA