Checkout (Google) "ZB2L3" or "Battery Capacity Meter Discharge Tester
Analyzer". (Less than $10, 15 volts max, doesn't provide a log, just
shows capacity)

I used an old micro-USB phone charger to power the tester. (Low-power
is OK as it is not doing any charging).

For the load I used a couple of 7 Watt car taillight bulbs in parallel.
(The
resistors supplied with the tester are not suitable as they get too hot at

12 volts)
I mounted everything in an old metal toolbox.

Easy to use, cuts out at the voltage you choose, shows capacity in amp-
hours.

I've been using 6 of these for years, haven't had any problems.

I get much more repeatable results if I only test batteries that have
been left on charge for a few days after the 'full' light illuminates.
I suspect that lithium chargers may indicate 'full' a bit early while there

is still ~5% to go before they actually shut off.
I know that Gellcell / NiMh / Lead Acid chargers indicate 'Fully charged'
when the battery is around 85 - 95% full.

Lots of good info on general glider wiring practices at
http://aviation.derosaweb.net/presentations/#wiring

Particularly like the bit about fusing light current wiring so that it
doesn't
go up in flames. After a bumpy landing I once had an undercarriage-
warning wire let out some smoke. It was connected to a 10 amp fused
bus. The fuse didn't blow due to the resistance of the thin wire but it
still
got very hot! Luckily it was Tefzel wire and didn't melt.