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All About Batteries, Part 3: Lead-Acid Batteries
1/13/2014

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Terminal voltage verses duration of discharge for several rates.
Terminal voltage verses duration of discharge for several rates.

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Max The Magnificent
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Comparison Tables?
Max The Magnificent   1/14/2014 11:52:35 AM
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Hi Ivan -- at the end of this, once you've worked your way through every battery technology known to humankind, will you be presenting an overall comparison matrix showing the various technologies in relation to each other?

Duane Benson
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Answered before I asked
Duane Benson   1/14/2014 1:16:51 PM
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re "Sealed lead-acid technology is sluggish and cannot be charged as quickly as other battery systems" - You just answered a question I've had for quite a long time. This has been a good series.

Batteries are a bit of a mystery but too important to be left as a mystery.

antedeluvian
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Lead Acid Batteries
antedeluvian   1/14/2014 1:22:58 PM
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As Mark Twain might have said, Everyone would like to get rid of the lead-acid battery, but nobody does!

I have been working on a battery backup unit design for industrial power supplies (24VDC) for 12 years or so. There is an old English joke-

Q: How do you start a pudding race?

A: Say go (ask Max to explain)

The project has not gone anywhere because management has never allocated the resources, but the need remains. Part of the issue of the design is that you need to charge a nominal 24VDC battery with a 24VDC power supply. There are several approaches to this, buit our current proposal is a buck/boost switchmode power supply. Boost for when the battery exceeds 24V and buck for the other case.

Anyway one of the considerations is that in order to test the battery, I am told you need to drain the battery through a load. The problem is that once this test is done, the battery needs to be recharged- what happens if backup is needed during the charge cycle? Do you concur with this concept that lead-acid batteries be regularily load-tested?

 

antedeluvian
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capacity Mesurement
antedeluvian   1/14/2014 1:28:59 PM
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Recent Brad Albing blogged about a new TI chip that acts as a gas guage for lead-acid batteries. It looked good to an innocent like myself, and I was wondering if you thought the product had any merit. Looking at it, it does seem to need to apply a load to guage the remaining charge, but so much of the topic is new to me that I would like an expert's opinion.

David Ashton
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SLABs
David Ashton   1/14/2014 3:06:28 PM
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Hi Ivan....another informative article, thanks.  I know VRLAs as SLABs (Sealed Lead Acid Batteries) which in Australia can be confused with a box holding 24 cans of beer :-)  

Can you explain how the technologies for this overcome the gassing that is usual with the wet-cell types?

And how do the so-called Deep-cycle batteries avoid the sulphation that happens with ordinary batteries if they are deep-discharged?

Adam-Taylor
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Specification
Adam-Taylor   1/14/2014 3:33:41 PM
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A great series of blogs, when it comes to specifying a battery what are the cardinal points of the specification (technology, end of life votage, amp hour etc) that the engineer needs to consider.

 

eetcowie
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Re: Comparison Tables?
eetcowie   1/14/2014 9:03:50 PM
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...comparison tables..

Yes, at the end it will be useful to see it all, so they say. I'll decide a good format as I get closer to the end.

eetcowie
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Re: Lead Acid Batteries
eetcowie   1/14/2014 9:26:42 PM
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" ...in order to test the battery, I am told you need to drain the battery through a load. The problem is that once this test is done, the battery needs to be recharged- what happens if backup is needed during the charge cycle? Do you concur with this concept that lead-acid batteries be regularily load-tested?..."

Yes, to test it, you must do a load test (but the load you use is your own design). Lead-acid is notorious for looking fine at the very end of its service life; that is, until you need to get some juice from it. It is so important, that alarm systems and uninterruptable power supplies (UPS) will do a load test periodically, to verify that the battery is viable. In many backup systems by APS for example, the load test is substantial, and the unit shuts down the output for a few seconds to complete the test, everytime the unit is plugged into AC power, and everytime the power-on button is pressed. In alarm systems, and some emergency-EXIT signs, the test is scheduled. For these systems, the output is always live, and only done while there is AC power, to have continuous/uninterrupted service. Float charging replenishes the load-test energy. For those systems that are always completely battery powered, the load test is very light, and lasts an extremely short time, so that the battery will not be overly drained. This can be designed to keep power to the load duriing the test, also. Designers will select a battery that is a higher capacity, to just account for the extra energy that load-testing drains during its service life. Once a very specific battery is chosen, its characteristics are well known as it ages. Therefore, it is possible to do a light load for a very short time interval, and know the battery is ok -- especially if the device has a real-time calender to tell you how old the battery is. Some smart batteries include this function.

eetcowie
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Re: capacity Mesurement
eetcowie   1/14/2014 9:36:19 PM
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"Recent Brad Albing blogged about a new TI chip that acts as a gas guage for lead-acid batteries. It looked good to an innocent like myself, and I was wondering if you thought the product had any merit. Looking at it, it does seem to need to apply a load to guage the remaining charge, but so much of the topic is new to me that I would like an expert's opinion."

The chip is very nice. It turns on a MOSFET periodically, to measure the battery terminal voltage. When it does this, it draws a small extra current, which is compared to [their proprietary piece]; doing this allows the internal resistance to be measured (see my previous blog on this). See my other replies on this topic, about the need to do load tests on lead-acid.

eetcowie
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Re: SLABs
eetcowie   1/14/2014 10:05:01 PM
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"Can you explain how the technologies for this overcome the gassing that is usual with the wet-cell types?"

looking at the chemical reaction, the GEL is esentially absorbing the Hydrogen, before it can accumulate into a bubble. But in addition, the chemistry/charge-voltage will not produce enough at a rate high enough, unless the cells are overcharged. You will see a warning put out by many manufacturers about popping the valve and explosion hazard, for overcharging due to Hydrogen outgassing.

"And how do the so-called Deep-cycle batteries avoid the sulphation that happens with ordinary batteries if they are deep-discharged?"


Some manufacturers use an additive, that disolves elemental sulfur, keeping it in solution. Others modify the plates to alter the ion density, to slow the movement of the heavier lead-sulfur crystals so it cannot stick. Another clever approach (specialty battery) is a mechanical structure that monitors the specific gravity and adds a buffer from a reservoir, which reverses at full charge. Deep cycle batteries need to be recharged fully, and done soon after the deep discharge is over. Self discharge is the enemy for sulfation. Folks don't realize the trade-offs that deep-cycle causes, such as lower cycle life, lower pulse-current ratings, etc.

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