Re: Battery Testing
 

Alan is right. As a rechargable battery is used over and over the discharge curve drops down.

so the voltage for a new Exide KOP battery with a 20A load might be, lets say... 11V when it is 50% discharged

and for a battery that is a year old, it might only be 10.5V when the battery is 50% discharged.

So, yes, you would have to run each battery your team owns through a charge/discharge cycle to get the present discharge curve, and print it out for each battery.

Battery remaining-charge meters are very complex. They have gotten pretty good on newer laptops, but I remember the older laptops: the little battery indicator went something like: 100% for several minutes, then 80% for an hour or so, then it would drop to 10% about 20 seconds before the laptop died.

Re: Battery Testing
 

The only this I want my device to measure a 'fully' charged battery's discharge curve when putting the battery under a large load. Then if you make some assumptions you can come up with an idea of how good your battery is.

Im still thinking about it. I need to understand more about our batteries.

I know we can't test the plates for how much build up are on them. Does plate build up cause internal resistance and/or less capacity, both rite.

Tell me if my thoughts are wrong on this. A battery trys to keep a constant voltage rite? But when it has less energy and gets hotter some of the voltage drops on the batteries internal resistance, making it even hotter. If you measure the voltage curve couldn't you predict the drop across the battery and find how much build-up is on the plates? I haven't done much with batteris. The only stuff I know is really from chem class and physics class but thats not practical.

Re: Battery Testing
 

the jpeg below it the discharge curve (typical) for a lead acid battery (like the ones in the KOP)

to measure the discharge curve, you have to run the battery through the entire discharge cycle.

as you can see, if you pick a part of any of the three curves, you can find a part on the other curves that look very much the same, so if you only measure part of the curve it would be difficult to tell which one you are on (the new battery curve or the old battery curve....)

but I would not rule out that it could be done with a quick test (a few minutes maybe) to get a general indication of the state of the battery.

You are correct in your understanding. The cells of a lead acid battery put out 1.5V each, and that voltage does not change much as the battery is discharged. What does change is the resistance of the electrolyte, and the contact of that liquid to the plates. In a way, when a battery is used its like the battery is getting smaller and smaller, until the cells start to drop out (electolyte is closer to water than acid) and they can no longer generate a voltage.

Re: Battery Testing
 

What we have found is that there is little change from new to old batteries. The internal resistance remains relatively constant as does the terminal voltage under load at least at 7.5 amps. Even the amp hour capacity changes little until the battery is well used (several years is typical). When things start to go bad they go in a big hurry. Sudden drops of 1/2 the amp hour rating over a few days is common.

Re: Battery Testing
 

In looking to upload a picture of the test overlays, I found a previous posting. See...http://www.chiefdelphi.com/forums/sh...4&page=2&pp=15 for a graph and explanation. It is the last post.

Re: Battery Testing
 

You can easily make a nice battery load tester with an old robot
controller and a few of spike relays controlling loads to high
power resistors that can be bought from digikey. Use a simple
voltage divider to read the battery voltage directly with the robot
controller. Run a load similar to the average load that you expect
your robot to apply, 20 amps would really be a minimum, and plot
the voltage of the battery as a function of time in the manner that
Al has shown in the prior thread. When this discharge curve starts
showing unattractive behavior compared to your good/new batteries
mark the battery for practice use only. Keep the records on all of your
individually numbered batteries from year to year so that you can
identify a battery that has taken a turn for the worse.

We qualify our growing collection of batteries in this manner just
prior to every season. Yes, it is a lot of work and writing a RC
controller program that automates the process is a good idea.
The effort is worth it. It is good insurance against a battery
going bad during a competition.

You can build/buy a sophisticated constant current load tester,
but you can do a good enough job with simple load resistors.
Don't forget the cooling fans...

Have fun,
Eugene

Re: Battery Testing
 

There are a few things you can test on a battery. Common parameters of interest for FIRST teams are capacity (how much energy can it hold), State of charge (how much energy does it have in it at the moment) and condition (how much of the maximum energy it can hold will it deliver - or in other words how 'good' is this battery). All three use different tests.

Ken is testing capacity with his 600 mAh NiCds. He needs to understand the conditions under which the battery will deliver 600 mAh - batteries of that size are tested at a 10 hour rate, meaning he should be able to get 60 mA for 10 hours (=600 mAh) before the battery voltage drops to some specified value (usually 0.8 volts for NiCd chemistry).

The Lead-Acid batteries used in FIRST are rated at 18 Ah, but large batteries like this are tested at a 20 hour rate, in this case meaning 900 mA for 20 hours. If your discharge rate increases, say to 50 Amps, you might see 5 minutes before the voltage drops to 7.5 volts, or a bit over 4 Amp-hours (Ah). Discharge rate and final voltage are key parameters here. This measurement is not very critical for a FIRST team, since we know what a new battery can do.

Capacity can only be measured accurately by a long-time discharge.

Condition can be measured fairly quickly, if necessary, or through a long-time discharge as above, in both cases by then comparing it to a new battery or manufacturer specification. But, condition is usually fairly constant, it changes over time but slowly. A FIRST team might want to measure all their batteries a week before competition, to pick which ones are for practice and which are reserved for competition. The quick test is to start a discharge test, but at a much higher rate (usually 3x the battery AH rating) for a fixed period of time, and comparing that to a new battery's behavior. There are microprocessor-controlled battery analyzers that do this without a large load, look up the company Midtronics. The aforementioned West Mountain Radio Computerized Battery Analyzer (CBA) is great for this purpose.

This kind of test is what your car workshop does to see if you need a new battery.

The final kind of test FIRST might be interested in is State of Charge, or how charged is this battery. A load test is not the correct method (it also reduces the charge considerably, defeating the purpose), instead a test under a small load is appropriate.

We need to consider a phenomenon called "surface charge", where there is a small amount of 'false' energy in the battery, a remnant of a recent charging. Let a battery sit for 60 minutes and the surface charge disappears, OR put a small load (I suggest a car headlight, 55 watts = about 4.5 Amps) for about 60 seconds to remove the surface charge.

Once there is no surface charge, a simple voltmeter test WILL tell you the state of charge accurately, but only if your voltmeter is accurate !!

12.64 volts is a fully charged lead-acid battery. You will measure up to 12.75 volts, any higher than that is surface charge. 12.50 is a 75% charged battery, 12.35 is 50% charged, 12.00 Volts is a 25% charged battery. (You will often measure voltages far below this - this is the opposite of surface charge, I call it surface discharge. Again. letting the battery sit for an hour will allow that to equalize, and you'll get an accurate reading.)(Note: Leaving a lead-acid battery below 12.50 volts for a long (days) time will damage it)

Notice the problem here? The difference between a fully charged battery and a 50% discharged battery is only a few tenths of a volt. To help differentiate between a 90% charged battery and a 100% charged battery (a difference of a few hundredths (!) of a volt), a small load (that same car headlight) can be used while measuring. I actually use a brake light bulb, 21 watts = about 1.8 Amps. This technique will show a few tenths of a volt difference at high (90%-100%) charge conditions, just what you are seeking. With the light connected and glowing, measure the battery terminal voltage right at the terminals (anywhere else will introduce error).

I could give you the exact voltages versus state of charge, but perhaps you should try this for yourself. Get a nearly brand new battery, or at least one in excellent condition, and charge it fully, allowing it to sit on a FIRST-type automatic charger at least overnight. Measure the voltage under a light (no pun intended) load. Then, pull off some capacity (900 mA for 1 hour brings it down to about 95% - maybe measure in 10 minute increments) and measure again. Make a chart, and you'll be in business. Maybe share with the rest of us...

Some other things about lead-acid batteries:

As lead sulphate forms on the plates, the internal resistance of the battery increases, which we call discharge. The process is not 100% reversible, so there's a little loss of capacity each cycle. Shallower cycles are more reversible than deep cycles. Eventually the battery will not accept a charge, it has then failed a 'normal' failure after a good service life (assuming nothing mechanical in the battery has broken). Allowing a battery to sit for any period while discharged (even a bit) causes damage. This means, a lead-acid batter prefers to be fully charged at all times. Heat hastens the normal failure mode, and is generated by charging and discharging (mostly due to ohmic resistance, not really by chemical action). Self-discharge can be about 20% per month.

So, keep your batteries charged as much as possible, always store them charged, and recharge at least every 2 months, more often is much better.

Hope this helps!

Don

(PS: I do batteries for a living).

Re: Battery Testing
 

According to the Excide spec for the battery, if the discharge rate is 54A then the battery should still be at ~10.8v at 5 minutes. It doesn't look like we hit the knee in the curve until somewhere around 7-8 minutes. Haven't check this on our batteries, but it is on our to do list.

The BCI (Battery Council International) indicates that a C/3 load (6A for the EX18-12) should be applied for 5 minutes followed by a 10 minute rest period for deep cycle batteries like AGM/GEL technology to remove surface charge before measuring SoC (or a rest period of at least 1 hour). For starting batteries such as (automotive type) flooded cells, the recommendation is ½ the CCA (cold cranking amps) for 15 seconds followed by 10 minute rest period. I suspect these are conservative recommendations – we’ll give the 4.5A @ 60s a try to see if that sufficiently removed surface charge.



I’ve been looking at information that indicates that deep cycle AGM/GEL cells have a different SoC table than flooded-cell technology. It isn’t much, but the temperature compensated table appears to have AGM/GEL cell voltage at a ~1.5% higher voltage (12.79v) than flooded cell which is 12.64v at 70F when 100% charged. This information seems to match what we’re seeing on our bank of 6 Excide batteries from the last couple years. Fully charged batteries that have been off the charger for a week are showing ~12.79-12.84v for batteries from 2005/2004. There is one suspect battery that is only showing 12.49v when fully charged, but it is from 2003.