Deep cycle battery capacity

• posted

Someone I know bought a 12V deep-cycle battery for their vehicle, for camping trips and they asked me a question...

I've often read comments like, "If you regularly discharge a deep-cycle battery only 40%, it will have a longer life than if you discharged it 60%."

Well... How do you tell how far you've discharged it? (How do you figure out what percentage capacity remains in a battery?)

Allan.

• posted

The battery has a capacity rating, given in Ampere-hours. You just keep track of the current you use, and how long you use it - when your use gets to half the advertised capacity, you should recharge the battery.

The A-H rating is normally based on discharging the battery to 10.5 volts, over 20 hours. If you discharge much faster than the "20 hour rate", you will get less than the advertised number of A-H.

```--
Peter Bennett VE7CEI
email: peterbb4 (at) interchange.ubc.ca        ```
• posted

The only sure way to measure charge state (eg capacity remaining) with a meter is to know a) The battery capacity and b) how much has been taken out. Ordinary voltage meters are quite inaccurate as the battery voltage can vary with age and temperature. A decent meter will measure the current draw and calculate how many Amp Hours have been taken out.

This is the best type of meter to use.

No lead acid batteries really like being deep discharged not even those rated for Deep Discharge in my experience. The fastest way to kill a lead based battery is to leave it to go flat. If possible buy a charger that has a permanant "Float charging" mode. These are designed to be connected all the time (24/7) - except when you want to use the battery obviously!. They have a slightly lower output voltage in this mode that's designed to stop the battery "boiling" like it would on a regular charger by keeping it fractionally below 100% full. Some have an adjustable output voltage so you can keep it just "off the boil". Just before you go on a trip you can switch it from Float to Regular chargeing to get the kast few % into it (but I don't bother). When you use the battery try and get it back on the charger as soon as possible. Don't leave it for a week. Keep the water level within limits and only use de-ionised water.

Colin

• posted

Anyone know of such devices suitable for use with small batteries 12V sealed lead-acid (7 to 10 amp/hour)?

In particular I'm looking for chargers that:

- will take 24V DC input

- small size - ideally < 3" x 3" x 3".

The batteries are hooked to some devices drawing about 100mA, so the charger would be supplying the current to the device when 24V DC power is available, as well as trickle charging the battery.

Anything commercial suitable?

Also interested if anyone wants to design & supply such units. Only looking for quantity of about 50 maximum though.

Dave

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• posted

sealed

charger

available,

and

The size requirement might make it tricky but otherwise ok.

The spec is similar to that for a regular DC-DC converter see "Charging lead-acid batteries with a power supply" on this page. Lots of other good info as well.

The battery manufacturer can recommend the exact voltages for your model of battery if you want best performance.

Keep an eye on the tollerance of the output voltage. 5% would be about 0.7V which is enough to make the float voltage too high.

Colin

• posted

Although not a definitive guide and I'm sure someone else will correct me, but I use a digital volt meter and measure the (offload) voltage at different times to build a rough picture of the voltages you expect when full/half/empty etc. As mentioned temperature etc will change these but as a rough guide a full battery( in good condition) will read somewhere 14.0 - 14.4v (more when 'hot' off the charger) When empty the battery would read 12.0- 12.5v say. You can then check the voltage/ charge level using the meter ( when no current is being used ideally) If you put the meter on whilst charging you'll see voltage come up slowly over the hours.

I was told to beware of the type of charger - in order to get the best life from the battery. If you constantly use fast chargers, 1: the life of the battery is shortened, 2: since the cells are pulled up above 14.4v 'boiling' occurs thus losing water longterm. Its better to have say a 13.8v fixed charger which will charge very slowly ( not usually a problem unless you're out every week caravanning~), thus effectively trickling the battery upto 85-90% capacity over 3-4 days. This type of charger will self limit at that voltage so there's no chance of overcharging the battery & you always know where 'full' is. Unlike say Nicad batteries which have a memory effect and don't like being just topped up anytime, a lead acid type battery requires the charge in the acid to stop the acid attacking/reacting with the plates, so its best to fill it up asap when any has been used.

As I said at the top, I'm sure others may disagree with some of this... so please feel free to correct me where appropriate..

Andrew

• posted

me,

full

slowly

life

the

the

As you suspect you are quite a bit off the mark here. Have a look at any of the SLA battery manufacturers web sites for technical details.

• posted

Also remember that all gel cells are SLAs but not all SLAs are gel cells.

```--
Aefauldlie, (Scots word for Honestly),
Robert, (Auld Bob), Peffers,```
• posted

Thanks for the info - quite helpful.

They do have a dedicated 24V - 13.8V intelligent battery charger, but this is unfortunately far too big.

They also have some really nice small modules which would all me to convert

24V down to 15V. I could then shove 1.2V worth of diodes on the output to give me 13.8V output. Not an intelligent battery charger, but I guess it can trickle charge the battery ok. I'll order a couple of them & see how they perform.

Cheers,

Dave

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• posted

These look smaller ...

24/12-5

49mm x88mm x68mm