Quote:
Originally Posted by Starkler
Not quite right.
The discharge rating is based by convention on a 20hr discharge period. ie a 20A/h battery can give one amp for twenty hours. 20A/h does not mean it is capable of 20 amps for one hour or even 2 amps for 10 hours.
You really do want to be conservative when calculating the amps budget and always buy bigger so as not to run your battery flat, which will shorten its life considerably.
A tip: When the unloaded voltage of an SLA battery is 12.0V you have drained very close to 50% of its capacity.
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Geoff's on the money there prova: there's also been quite a lot of threads/info of late in IIS on this very subject.
SLA's (along with others) perform and last best when they are "floated" or maintained at terminal voltages between approx 13 - 13.5 volts: effectively meaning that you should not heavily discharge them in useage before recharging. Also, it is important to realize that the "trickle charging" devices that come with the auto jumpstart units should not be left on permanently when the battery is not in use, only employed to bring the unit back up to full charge after use, or to re-float (boost) the unit for short periods every couple of weeks if the battery is stored for some time (allways storing it fully-charged.)
I operate an HEQ5 Pro mount and purchased a 17 Ah auto jumpstart unit for its' operation; these are also rated in "CCA" units (900 amps for the 17Ah) which is a "cranking amps" capacity rating because they are, after all, all automotive units. This 17Ah job runs the HEQ5 Pro adequately for a night's use, the mount drawing a couple of amps in use.
The greater the amp-hourage, the bigger and heavier the unit: you may wish to consider the next size up if you have intentions of drawing other loads (devices) from the battery other than just the mount operation.
As Geoff said, the 20 hour rating has been the auto industry standard since time immemorial: other deep cycle batteries (stationary etc) will have the 10 hour rating (not allways) and will allow a greater current to be discharged over this 10 hour period: eg, two batteries, each 100Ah with one at the 10 hour and the other at the 20 hour ratings - theoretically, you can draw 10 amps for 10 hours from the 10 hour rated battery, but only 5 amps for 20 hours from the 20 hour rated battery.
Geoff's comments about not being able to automatically extrapolate/infer different lengths of discharge times for various loads (current draws) are also correct, the lower the discharge amperage, the nearer, technically, you can get to its full 20 hour rating capacity.
Don't get a unit less than 17Ah (900CCA) and don't forget to recharge the battery after using:
don't leave it recharging beyond fully charged and don't use another charger with it unless it is a fully regulated one:
don't have the charger operating whilst using the unit to power the mount (unless it is fully-regulated, and don't even then!)
and don't forget to periodically recharge the unit if it is not being used regularly.
You should then have good service from the jumpstart jobs!
Please forgive my (usual) full-on rave here, being an old "battery man" there's the subconscious imperative to grab any opportunity to barf on on this particular subject!
Cheers, Darryl.
I charge the jumpstarter every time anyway, and i need to keep it in the car now thanks to dodgey battery....so i always have a backup
