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Author Topic: Identifying the ampage of a sealed lead acid battery  (Read 5609 times)

NFMike

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Re: Identifying the ampage of a sealed lead acid battery
« Reply #25 on: May 13, 2015, 02:40:11 PM »

No-one seems to have noticed the 15min on the end. I'd hazard that at 34W per cell (from full charge) it will hit 1.67V in 15 minutes. But I think I'd be wrong somehow.

Regardless of that, lead acid batteries should be charged with volts until they reach their fully charged voltage. Then they can be trickle charged. If you get a correctly sized auto charger (eg. one for say 5-20 Ah batteries (12V of course) then it should just do the job without you worrying about it.
Probably better to get one that is slightly oversize than under - it won't overcharge as the cut-off volts are not dependent on size but chemistry. Undersize may simply not have the oomph to get the battery up to the cut-off volts, so it never fully charges or terminates. (I had one that was like that from a popular supplier on here - pos. Had to get a better one from Greece of all places, via ebay.)

AlexC

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Re: Identifying the ampage of a sealed lead acid battery
« Reply #26 on: May 13, 2015, 02:47:59 PM »

Which Is why I stated that it can vary...

Quote
In theory
9Ah means the battery can deliver 1 amp for 9hrs
or 9A for 1 hr etc. etc.
But this can vary depending on the Rate of the battery (10hr rate or 20hr rate) which is determined by battery construction and intended use.

I agree, battery capacity claims do border on the fraudulent and the result is a lot of confused and unhappy people when the battery does not perform as expected.

It would help if manufacturers agreed a STANDARD for battery specifications which included the RATE, but I don't see this happening anytime soon.
Some are better than others in this respect but there are a lot who make some very wild claims.

As you say, they are relatively cheap and make useful ballast in our application, but that is no excuse for poor/misleading advertising data.

Keep Happy.

Sandy. :-))

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Calimero

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Re: Identifying the ampage of a sealed lead acid battery
« Reply #27 on: May 13, 2015, 02:56:52 PM »

Quote from: Inertia
Well said, Monsieur!
Far too many modellers are fooled by those seductive capacity ratings, to the extent that some believe they can even run two brushless motors from one SLA battery and obtain an hour's sailing from one charge. If I've seen one Huntsman or Perkasa lumbering round a pond like a narrowboat, weighed down by a huge brick of a battery, then I've seen a hundred. It makes me weep. It takes a great percentage of the motor's power just to get the battery moving, never mind getting the model up on the plane.

Anything faster than a tame tug is indeed NOT suitable for SLA batteries.

I run my springer (with a big nice #174 45mm prop from Raboesch) with a BLDC motor off a 6V SLA """7.2 Ah""" battery. When cruising around, total amp draw is around 2.6A which would give me almost 3 hours of constant run time. Effective run time (because you're not always cruising all over the place) is 4 hours +.

But when I start pushing my 25kg-shoe-box-shaped barge or towing a heavy ship around, current draw goes through the roof: 11A at full throttle.  Datasheet says run time would be around 25min with a constant 11A load. Actual run time is closer to one hour. I was actually surprised the first time I took my barge to the pond. I had somewhat forgotten that SLA batteries were so "weak".

Quote from: Inertia
Unless they are of the special high-current, deep-discharge "leisure" type

Even the "deep discharge" - at least those that are affordable - perform only marginally better. I'm tempted to look at LiFePo4 batteries, for that matter. They seem to offer a good balance between "usability" and discharge/charge rates. And they are getting cheaper.

I have two 12V / ""12 Ah" batteries in my Odin tug. Good enough as the overall power draw isn't that bad and I needed around 13kg of ballast anyway, but this is as "ideal" as it gets.

On smaller boats, SLA batteries might not be that good a choice.
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AlexC

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Re: Identifying the ampage of a sealed lead acid battery
« Reply #28 on: May 13, 2015, 03:50:32 PM »

No-one seems to have noticed the 15min on the end. I'd hazard that at 34W per cell (from full charge) it will hit 1.67V in 15 minutes. But I think I'd be wrong somehow.

Regardless of that, lead acid batteries should be charged with volts until they reach their fully charged voltage. Then they can be trickle charged. If you get a correctly sized auto charger (eg. one for say 5-20 Ah batteries (12V of course) then it should just do the job without you worrying about it.
Probably better to get one that is slightly oversize than under - it won't overcharge as the cut-off volts are not dependent on size but chemistry. Undersize may simply not have the oomph to get the battery up to the cut-off volts, so it never fully charges or terminates. (I had one that was like that from a popular supplier on here - pos. Had to get a better one from Greece of all places, via ebay.)

Yes, I admit I did not spot that on the picture of the battery... I was going by the spec data in a later post.

That is exactly what it means...

Nominal cell voltage (fully charged) is 2 volts.
Fully discharged is typically between 1.6v and 1.8v depending on make.
2v at 34w discharge = 17 amps which would discharge the cell to 1.67v in 15 minutes.

Since the 6 cells in the battery are in series this would mean at a 17amp output rate the battery would be discharged in 15 minutes and be at a voltage of 10.02v... that is a discharge rate of 1.88C

Also on the battery picture I can just make out the max initial charge current of 3.4A at a constant 14.4v - 15v charging voltage... which may require you to adjust the current limit on your charger... or risk generating gasses which could damage the cells.

Now it all makes sense.

Keep happy.

Sandy. :-))
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