Cant work this out

[GW8IZR]

http://www.amazon.co.uk/cars/dp/B0015PI7A4/ref=sr_1_1?ie=UTF8&qid=1437803405&sr=8-1&keywords=solar+battery+tester

Not a crude load tester, this will give you a very good indication of what your battery is like.

HTH

[Boydie]

Looking at both of the electronic devices, the simple hydrometer is a lot, lot cheaper and can tell you which battery cell is playing up or low on electrolyte.

[GW8IZR]

Well fair enough but forgetting the practicalities of dealing with gel cells and sealed batteries I'm not sure why you need to know which cell is down as in practice once you get to that stage its not long before a short one way trip to the scrap man.

Using a swept ac source to measure the conductance is the only practical way to predict remaining service live - unfortunately those tools are very expensive. A cheap single frequency device as per the link above is remarkably accurate by comparison. (edit) even without knowing the current state of charge which for breakdowns is a bonus.- the swept frequency units can even test on load or during charge.

As solar energy and battery storage is getting more common the testing of batteries is much more of an everyday thing and I predict some clever, cost effective devices are on the horizon.

In the eighties I used hydrometers for telephone exchange, site standby batteries and trackside NiFe cells for signalling, the cost of replacement and damage to equipment property and lives meant the prediction of end of life was very important, more so than the inconvenience of a flat car battery - although in your desert escapades that would of course be a serious risk.

Whilst I managed with a hydrometer and graph paper the time taken to test and log was huge, I wish I'd had a swept conductance meter or even a single frequency device, the world has moved on.