No more flat battries from your cool-box, TV, lights, etc?

[haydn callow]

You will never get to use the full amp hour capacity of any battery. Lets say we have a 100 amp Hr new battery. After a very short time you will never get the 100 amps back into it. Most battries in good condition charge to about 90% capacity and the bottom 40% is useless. This gives you about 50% of your battery which you can safely use. If you discharge a battery below 11.8 volts you will shorten it's life very quickly. Most decent inverters cut out when they sense 11.8 volts. They do this to protect your battery. Any battery that is discharged much below this will be damaged to some extent. On my narrowboat I had 4x120 amp hr leisure battries and because of the way they were used they lasted about 18 months. They never went below 11.8 volts. People who ignored this were changing battries after 6 months or forever trying to charge. Never let a battery go below 11.8. I doubt you could start a bongo at 12 volt or less.

[phedders]

Are you measuring that 11.8v under load or quiescent though - that was my question. You are so right that you can (should) only use 50-60% of a cranking lead acid's "capacity" though other types are much more forgiving... My Hawker cyclons have been very very abused over 11 years... often being discharged to no measurable voltage (through neglect) or discharged down to 6v or so, repeatedly and still charge to pretty much the 25Ah they started with.

A propper traction battery with big thick plates is also going to fare much better with deeper regular discharges. A cranking battery with thin plates will warp and the plates nearly vanish if you use anything like their 'full' capacity...

By the way - my Bongo was starting with the battery down to even 11.8 and 11.7 when I got it - and all because it wasn't getting a good charge because the terminals were dirty/corroded. When the glo's were powered up the voltage dropped to under 11... I was surprised it was starting at all!

[johnzbenson]

I agree with the previous post. the battery saver is exactly what it says it is. If it cuts out at 11.2 volts under a load then with no load on it with avoltmeter it will probably read about 11.5 volts. which may not be enough to heat your plugs and get you started which can only be adjudged by a true battery condition check (this is where the garage puts a high load voltmeter across the terminals and watch the battery voltage decline oh and the test device should get hot)
But essentially it does not allow you to discharge down to absolutely flat and only a light recharge should be needed to return your battery to full charge.

As a final note if you fully charge a battery by normal means ( the alternator delivering between 13.8 to 14.4 volts to the battery in normal driving and there are no excessive loads placed upon it) if you disconnect the battery and then test the voltage the battery will read in the region of 12.25 volts perhaps up to 12.75 volts (leave for ten minutes after charging to test)

so whats the message ? for a fiver it will save the battery and may save any embarrassment getting jumpstarted ( but not Guaranteed) the prefered route would be a leisure (deepcycle) battery with static switchover -only switches into circuit for accessories when ignition off and prevents the main batttery from being discharged. but hey for a fiver it can't hurt.