Here is a nifty, plugin battery meter:
http://www.comparestoreprices.co.uk/car-alarms/unbranded-car-battery-meter.asp
[I can't quickly find a link to a US source, but....]

I have 2-3 of these.  They were first sold under the Escort brand; now
more generic.  Anyway, DON'T BUY ONE(!)

Using these sent me down a path of error--believing that battery
voltage was temperature dependent.  For example, just now one of
these says the battery in my Syncro is 10.3 volts or so.  This at
a temp of about 35 degrees F.  What I finally learned is that
if I use my digital voltmeter, it will tell me 12.5.

These nifty gizmos are extremely temperature dependent.  I don't know
why, but if they are not at 70 degrees or so, they are worthless.

[I finally learned this because I became involved with the photovoltaic
light system at a Colorado Mtn Club cabin at 10,600' in the mtns.  The
batteries there are 15-20 degrees F this time of year.  I thought,
because of this nifty gizmo, that they should read 10.x, blah, blah.
Well, no!  At 15-20 degrees, the 12.67 volts of a fully charged
battery will read 0.3 volts lower, so most solar people ignore
temperature dependencies all together.  So instead of buying one of
these at $15 (?), go to WallyWorld and buy their digital voltmeter
for $12.95 that will give you correct readings.]

Anybody want to buy one of these neat gizmos from me for $20?  ;-)
Or should I put them on Ebay?

                                Richard A Jones
                                Boulder, Colorado