Batteries and Battery Tender Information

[Ron]

Batteries&TendersFAQ.pdf

The first four pages cover a lot of forum FAQs.


Lots of handy/interesting info:

A battery in a rest state measuring 12.15V is only 50% charged.
% Charge = ((Measured Battery Voltage – 11.4 volts) / 1.5 volts) x 100

[Bitsyncmaster]

I don't agree you can calculate the charge of the battery by measuring the voltage. The only way I would know an accurate state of charge is to charge it and measure the power it took to bring it up to full charge.

As far as knowing when the battery is going dead. I would try the charge after it sits fully charged for a week or so. You can also test the amp/hours by discharging the fully charged battery with a known load and timing it. But that is not a good idea for lead acid batteries.

[Ron]

I was skeptic of that too, even after reading the whole paper. So, I surfed around looking at similar industry FAQs and, if mentioned, they all seem to agree, except one. They say 12.24 is 50% (FWIW, a deep cycle battery).

As far as knowing when the battery is going dead, they agree with you in the latter pages (except they say 24 hours, with several hours being fairly accurate).


Obviously, most of them are trying to sell something, but I'm sure they are aware that all this could cause returns if not true.

That part is interesting anyway....

[Bitsyncmaster]

"So a 12-volt battery will measure at about 12.9 volts when it’s fully charged
and about 11.4 volts when it is fully discharged"

Has anyone ever seen 12.9 volts on a battery that has been sitting?

[Ron]

I haven't. Never paid that much attention...First thing I do is throw a load test on it and if it passes, throw it in the car (I know, I know ).
But keep in mind they are talking about a brand new battery, charged 'perfectly', and then allowed to sit.

What I found the amazing is how long they claim it takes to charge one "correctly"...and that half of the required time only gets you to 50%.

[Bitsyncmaster]

Next person to buy a new battery, put it on a charger and read the voltage after it is fully charged and has been sitting for an hour without the charger.

You should charge a new battery anyway unless your stuck somewhere. I also charge them and it takes a good 20% of the battery capacity to reach full charge.

[Ron]

You should charge a new battery anyway unless your stuck somewhere.

(I know, I know ).