Hi Tony,

http://www.yuasa-battery.co.uk/industrial/products.html    C= the Ahour capacity

I see no easy way if your charger is a crude type with transformer,diode bridge,2 lights and not much else. A very rough guess is when cool and off-line is OK at 6.6 to 6.9 volts …….Voltage readings are affected by internal temperature and age.

If your charger is a "two-step" type with a regulated bulk charge current you can can calculate.

That type of charger  will step down the voltage,and maybe indicate with a light when it is 90% full.The point at which it steps to float/standby charge is when the current falls to C x 0.03(according to the 12v7Ah data sheet I browsed) That means the 90% figure depends on a suitable charger i.e don’t put a 7Ah battery on a golf cart or car charger in case it inputs 5 amp at 14.5 volts.

If you therefore figure the charge efficiency is 75% then 5 hours at the C / 5 rate would be enough for a partially discharged battery.

The fastest charge current for a Yuasa 12v 7Ah is C divided by 5 and ,if I remember, I had a charger that did a 6v4Ah at 400mA which is  C / 10 rate.

Note

The maximum charge rate is higher for deep cycle batteries than the series optimised for burglar alarms.

I noticed a few caddy-car charger manufacturers are saying disconnect after 24 hours. That ,I think, is a standard quotation and possibly because there is no benefit despite the float mode being OK for the Battery health onpermanent standby.

Thomas Screwinbulb

Ashley , I have seen that happen before.

If you have a reliable meter ,have confidence in what you are doing you could browse the internet for pdf files and fix your charger accordingly or use the C /5  C/10 and Cx 0.03 values.

A  12 /7 is the smallest I could find 12v 7 Ah datasheet

That  is not an issue to me………….When a golf supplier changed from standard 12v 24A to a deep cycle battery it was recommended that users tweak the float voltage down to 14.1 volt or bring in their charger to workshop.

If there are internal rotary pot adjusters ,even if you are working blind, you could first make a note of the  bulk charge current and voltage and the float voltage and threshold current  at which the step-down occurs.Muck things up and you can revert to the previous settings.

On the other hand,when your red light comes on it may indicate trickle,not float….100 mA is trickle charging a 4Ah but not a 30 Ah battery.

Tony, I was slightly vague in my first post : I didn’t mean to imply you could put a 6 volt battery on a 12volt charger .

Hi Tony,

 I’ve serviced chargers yet the Yuasa datasheet seemed gobbledegook until I stared at it for 2 minutes.

The C value is the battery capacity in amphours.

You will find the answer to your question in the charger case its packaging  or instructions {plus a little arithmetic }.The charging process is not 100% efficient.

If the charger is electronic and matched to the battery it will charge in 5 to 15 hours.That depends in the battery capacity and charge rate.

Regards

Tom