Any thoughts about the best way to test a lead-acid gel cell? I am plottin g the construction of a device that will discharge them to 11 V with a cons tant load and record the time it takes. Does anybody make that gadget comm ercially at low cost? Any shortcuts? Does testing under load tell you muc h?
Just toss a car headlamp on it and check back every 15 minutes or whatever.
Really, when was the last time anybody said "Whoah, I had no idea that battery was not working, I was fooled the entire time, even though it wasn't charging or discharging correctly!" A bad battery is pretty east to spot.
See my comments in resonse to your original question.
Also, for SMALL gel cells, I use a West Mountain Radio CBA-II to test batteries: The typical 12v 7A-hr UPS battery is about the limit of what I want to try with a discharge test. The heat sink gets rather hot and the fan works overtime.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Thanks. Yes, a dead or severely weakened battery is easy to spot, but I wou ld also like to catch less severe deterioration that would give less-than-e xpected battery life in the field. (I'm no longer talking about UPSes here , but rather, batteries for portable scientific equipment.)
Actually, it's not easy. I've been fooled too often. In my never humble opinion, the only way to do it right is with a controlled discharge test and comparing it with a known good battery. There's quite a bit of variation in the curves depending on discharge rate, battery quality, and thermal management making interpretation and analysis difficult. I've found that the only way to be really sure is with a comparison. Sure, a dead battery is easy to spot, but trying to predict how long it will last, whether it should be replaced, and how much life is left, are not easy. That's why large organizations do not test and replace UPS batteries on a fixed schedule.
That's what the CBA-II and similar discharge testers will give you. Such testing is common in the RC (radio control) groups.
Incidentally, when you buy SLA, gel, or AGM batteries, look at the weight. The better batteries will have more lead and therefore weigh more. The cheap junk that doesn't last more than a few charge-discharge cycles will weigh less.
You might look at Li-Ion replacement batteries for UPS's. I've only done one, about a year ago, so it's too soon to tell if it will survive. So far, no surprises. However, I cheated. I disarmed the charging circuit, and replaced it with a balance charger borrowed from the RC crowd: A balance charger charges each 3.7v cell individually. Another catch is that the nominal voltage is 11.1v, not 13.6v.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Thanks again! An 11-volt Li-ion system would not power my astronomy equipment properly. I do OK with gel cells but need to know, before going out in the field, that they are in good shape.
BTW, any thoughts on maintaining a gel cell by keeping it attached to a car's electrical system? What should I put in between? The car runs at 13.9 volts, which is a little high for float-charging a gel cell.
Thanks, looks promising; actually I might make such a thing myself.
I have a programmable power supply and programmable load fixture. I've messed around with NiCd, NiMH, Lithium, Lead batteries.
The most consistent/useful metric seems to be the internal series resistance of the cells...except for the obvious case where a shorted cell has low ISR ;-)
I've played around with using Bob Parker's ESR meter kit for testing batteries - the results appear to be significant, but of little use in our shop as we don't deal much with rechargeable battery products (like forklifts, etc.).
Notes here:
If anyone has readings to add to the page please send to me so I can expand it a bit...
John :-#)#
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Isn't the proper tech term AGM batteries?
We have a Harley with a 13.6VDC Li Ion battery. It has a maintenance charger with current for each cell. Shorai brand. Very light.
Not that high. Automotive electronics run on 13.6 VDC. A lead acid battery is about 2.2VDC per cell.
If your unspecified astronomy equipment is that critical, you're going to have problems with any type of unregulated power source. You might want to investigate what device is crapping out at low voltages and do something to compensate.
It's called a "charge controller". For charging from 12v, the RC (radio control) crows has the best stuff. This is typical: I suggest you watch the video (10min). 10-18v input. It will charge almost any type of small battery, including 1 to 12 lead-acid cells (at 2v each). Programming is ugly but once saves in memory, you're done. This is probably overkill for a gel cel but will be handy when you give up on gel cells and switch to Li-Ion. Note that this model will measure the battery ESR.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
What voltages do you normally measure across its terminals? I've seen various schemes used to get 13.6v. Some are lazy and just deliver
11.1v while claiming 13.6v. Some have a built in PWM (pulse with modulator) that acts as a voltage regulator to deliver the 13.6v. Some use 4 cells at 14.8v that barely reached full charge because that would require about 16.8v charging voltage. Hopefully, your also has short circuit protection.-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Are they the ones who taught Dumbo how to fly?
Also, the battery temperature has a huge effect on the ESR. I prefer the discharge curve mostly because I don't believe in a single number can totally describe the battery SOH (state of health).
As for building a discharge battery tester:
There are plenty of commercial discharge testers: but the West Mtn Radio CBA series seems to give the most useful results. However, the design does have a problem. It doesn't use a 4 terminal Kelvin connection for the leads. In the CBA-II, the battery voltage is measured at the tester, not at the battery terminals. If there is any voltage drop in the leads or the battery terminal connections, the voltage drop is going to mangle the graphs. It's not a problem at low discharge currents, but at the high currents expected from a gel cell, it will be a problem. I looked into modifying mine. I think it can be done but not easily, so I decided not to risk it.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
No. Dumbo will fly without the help of RC or crows?
Crows? Did I write that or was that my spelling chequer? It should be "crowd".
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
As I understand it, "gel cell" != "AGM".
Again, as I understand it, a true "gel cell" uses an electrolyte which contains a gelling agent. It's one step away from a flooded-cell battery; the gel keeps the electrolyte from spilling out if the battery is operated in a non-upright position. Think it it as a Jello-based battery :-)
"AGM" = "Absorbed glass mat". It's a form of "starved electrolyte" cell, in which a smaller amount of a more-concentrated electrolyte is soaked into a fiberglass separator mat located between the plates. As with a gel cell, there's little or no free-flowing liquid to spill out, but the mechanism used to control the electrolyte position is different.
It wouldn't surprise me if a lot of what people refer to as "gel cells" (e.g. the rectangular batteries for UPS applications) are actually being manufactured using AGM designs these days.
I have recharged a 12 V, 7 Ah gel cell by connecting it to a car cigarette lighter socket while I was driving, and it seemed to work OK. I just did it for a couple of hours on a couple of trips - I didn't leave the gel cell battery connected to the car as a regular thing.
You might want to put in a diode, to ensure that the gel cell doesn't try to supply the car's electrical system if the car's voltage drops for some reason. Something like a 1N5822 is good for 3 amps of current and will only drop about half a volt. The diode should "point at" the gel cell.
If you really don't like losing 0.5 V in the diode, you could wire in an automotive relay between the gel cell and the car. The contacts connect the gel cell to the car, and the coil is wired to the hot-in-run wire from the ignition switch. That way, the gel cell only charges when the key is on. You'll still lose a little in the relay, but much less, like
0.1 V or so.
Power-Sonic wants to use 13.50 to 13.80 V for float charging their batteries, so you're not that far off. On the other hand, your car's electrical system can be anything from about maybe 9 V (running the starter) to 15 V (cruising down the Interstate on a sunny day).
*If you can find one surplus*, you could use a DC-DC converter "brick" to float charge a gel cell. The ones you want have an input voltage range of 9 to 18 V DC (or wider), and output a constant DC voltage. A couple of problems: lots of these output 12.0 V exactly and don't have a way to adjust the output voltage up or down. Some are adjustable, though. Also, if you buy these new, they are really expensive for some reason - once you get over a couple of dozen bucks, you're probably better off buying a real official charge controller like Jeff Liebermann posted.Sometimes you can find power supplies that are meant to run laptops in a car. These often have a switchable output from about 12 to 20 V or so. You can set the output to maybe 14 V and use a diode in series with the output to get rid of another 0.3 V and have a fairly constant voltage to charge your battery with. Again, finding a surplus or closeout might help. A few years back, Rat Shock was closing out a
40 or 60 W version of one of these for $10, when it had sold new for $50+.It's sort of an offense against thermodynamics, but you could also use a 12 V DC to 120 V AC inverter, and then a regulated, switching "wall wart" DC supply to provide a constant voltage to charge the battery. You could even use a real official gel cell charger - these are basically wall warts with $1 extra in components, but they sell for 3 times the price. If you are doing stuff "in the field", you probably already own the DC-to-AC inverter, and if you luck into a wall wart with the right output voltage, this can be an inexpensive option.
Matt Roberds
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