battery life

[angryman]

If I have 2 7ah 12V batteries wired in series, am I correct in assuming that I will get 24V and 14ah? Sorry if that's a stupid question...

 

[sreid]

There are no stupid questions. But you'll get 24 V at 7 AH.

 

[Skogsgurra]

Correction: There are lots and lots of stupid questions.

But this one was not.

Gunnar Englund

http://www.gke.org

 

[angryman]

thanks How come if the battery capacity doubles, the current supply doesn't?

 

[OutToLunch]

the battery capacity did not double, the voltage did. The same amount of current will be drawn from each battery so the amp-hour rating will not change. If you put them in parallel, then the amount of current drawn from each battery is halved for the same load, effectively doubling the amp-hour capacity of the battery bank.

 

[angryman]

basic electronics...see....stupid question thanks!!

 

[angryman]

If I am using a battery charger to charge 2 12v batteries in series, it would see it as 1 24v battery, right? Would the fact that I am charging 2 12V's rather than 1 24V have any effect on the charging time?

 

[BrianG]

Your charger has to match the battery voltage. Connecting two 12v batteries in series requires a 24V charger - if you connect them to a 12V charger each battery will only charge to 6V! Note: charging batteries in series is not a good idea unless the battteries are identical types and age, and have the same discharge state.

If you only have a 12V charger you must connect the batteries in parallel. Charging time depends on the current your charger can supply and what is a reasonable rate for the battery in question. If your charger has a maximum or limited current then it will be divided between the two batteries so the charging time for the pair is doubled.

 

[angryman]

incidentally, how can I calculate charge rate/time...the only facts I have are that the batteries are 7ah 12V (2 in series) and the charger is a 60ah 24V...

 

[waross]

Batteries in series should be balanced as to type, size, age, charge level, and condition.
Re the amp hours;
12 volts at 14 amp/hrs = 168 watt hours.
24 volts at 7 amp hours = 168 watt hours.
respectfully

 

[BrianG]

OK, so you have a 24V charger: are you sure it's not a 60 AMP charger (not 60ah)? Sounds like a truck battery charger, so 60A would be the maximum current that the charger can supply. It is likely to be rather excessive for a 7Ah rated battery and would need a power limiting resistor. Have you got any data on the batteries to see the recommended charging current?

 

[angryman]

Using some calculators I'm finding online, I'm getting about 3 hours for a 7ah battery to charge @ 3A....I can't find a formula or rule of thumb to go by though...

Also, the 2 12v batteries are connected in series, and to my knowledge, are identical....

 

[angryman]

the charger is rated 60ah at 3A/24V....I have never physically seen the batteries, but I believe they are radio shack bricks.

 

[angryman]

uh oh...just looked at the charger again....60ah is the max battery size this thing will charge.

So I'm looking at charging a 7ah 24V battery with a 24V 3A charger.....

 

[BrianG]

Battery charge rate is usually expressed as a fraction of the capacity C in amp-hours, so 3 Amps is about C/2. A useful rate I have seen quoted for smaller batteries is C/5, so for a 7Ah battery 1.5 amps will recharge it in about 5 hours. Remember: the faster you charge it the less efficiecy and the hotter it will get and gas more. Really high currents will damage the plates in the battery and can cause short circuits in individual cells.