phil@diablo.amd.com (12/02/89)
I just found a source of 12 V 6 AH sealed no maintenance lead acid
batteries for $10 each. They seem to have a lot of applications (house
burglar alarm, CB radio power, video camera and lights, etc) so I am
looking for some advice on what a charger needs to do. What is an
appropriate fast charge rate? Is it ok to trickle charge them in a
burglar alarm application? What does the discharge curve look like?
How should a fast charger know when to turn off? Do lead acids have
the memory problems of NiCads? Is it ok to leave them flat? What is
the best condition to store them in?
And out of curiousity, how do the corresponding NiCad systems work?
I'm used to overnight, 1/10 rate chargers but lately I've seen a
Norelco cordless shaver that magically recharges in about an hour, as
well as a Makita 9.6V systems that recharges in about the same time. I
am told the Makita just dumps energy into the battery and stops when
it gets hot; it certainly does get hot. The cordless shaver doesn't
seem to get hot but it might be doing so inside, I think it stores a
lot less energy.
--
Phil Ngai, phil@diablo.amd.com {uunet,decwrl,ucbvax}!amdcad!phil
AT&T Unix System V.4: Berkeley Unix for 386 PCs!raoul@eplunix.UUCP (Otero) (12/03/89)
In article <28220@amdcad.AMD.COM>, phil@diablo.amd.com writes: > I just found a source of 12 V 6 AH sealed no maintenance lead acid > batteries for $10 each. They seem to have a lot of applications (house > burglar alarm, CB radio power, video camera and lights, etc) so I am > looking for some advice on what a charger needs to do. What is an > appropriate fast charge rate? Is it ok to trickle charge them in a > burglar alarm application? What does the discharge curve look like? > How should a fast charger know when to turn off? Do lead acids have > the memory problems of NiCads? Is it ok to leave them flat? What is > the best condition to store them in? Yuasa batteries has an excellent catalog, data sheets, and source of information on how to treat sealed lead-acid batteries. Call them at (800) 423-4667 or (800) 962-1287 for it. In general, recharge at a constant current of <1/10th the AH rating. Put a diode between the battery and your charger to prevent accidental discharge, and make sure your maximum charging voltage is limited as well. A CVCC (constant voltage constant current) recharger is your best bet. There is an excellent recharging circuit shown in the National Semiconductor data books for the LM317 and LM337 voltage regulators. Just make sure your heat sink is big enough as well. If you send me an address, I'll send you a diagram of the automatic recharging circuit I set up for some medical instruments here. Don't leave them flat, and do leave them charged in a cool well-ventilated place. They don't have Ni-Cads memory problems, fortunately. Trickle charging should not hurt a sealed lead-acid. -- Nico Garcia Designs by Geniuses for use by Idiots eplunix!cirl!raoul@eddie.mit.edu
henry@utzoo.uucp (Henry Spencer) (12/03/89)
In article <28220@amdcad.AMD.COM> phil@diablo.amd.com () writes: >And out of curiousity, how do the corresponding NiCad systems work? >I'm used to overnight, 1/10 rate chargers but lately I've seen a >Norelco cordless shaver that magically recharges in about an hour, as >well as a Makita 9.6V systems that recharges in about the same time. I >am told the Makita just dumps energy into the battery and stops when >it gets hot; it certainly does get hot... You can recharge nicads at blinding speed if you are equipped to monitor the temperature or internal pressure of the cells. The problem is that it's difficult to tell whether nicads are fully charged by looking at their electrical behavior, and high-rate overcharging causes overheating and internal pressure buildup. Military nicad systems often have sensors for pressure or temperature to permit rapid recharge. It should be possible for shavers and the like if the nicads are custom-made for them. Exactly how the shavers do it, I'm not sure. I took my Philips shaver (Philips products are marketed under "Norelco" in the US for some reason) apart a while ago out of curiosity. In contrast to my old one, which was about as simple as they come, this beast is pretty smart. It knows when it's low on charge, it knows when it's fully recharged, and you can plug it into either 110V or 220V without flipping any switches. Turns out the thing has a zillion-legged IC inside it, along with assorted other cryptic components. Judging by the tiny little multi-winding transformer, the lack of any obvious 110/220 switching even internally, and the FCC-compliance sticker (!) on the case, I suspect it's a switching power supply (!!) managed by that mysterious IC. I didn't think to look at the battery to see if it had any extra connections for something like temperature sensing. -- Mars can wait: we've barely | Henry Spencer at U of Toronto Zoology started exploring the Moon. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
kris@pnet02.gryphon.com (Kris Means) (12/06/89)
In article <28220@amdcad.AMD.COM> phil@diablo.amd.com writes: >I just found a source of 12 V 6 AH sealed no maintenance lead acid >batteries for $10 each. They seem to have a lot of applications (house >burglar alarm, CB radio power, video camera and lights, etc) so I am >looking for some advice on what a charger needs to do. What is an >appropriate fast charge rate? Is it ok to trickle charge them in a >burglar alarm application? What does the discharge curve look like? >How should a fast charger know when to turn off? Do lead acids have >the memory problems of NiCads? Is it ok to leave them flat? What is >the best condition to store them in? The July '86 issue of Radio Electronics magazine ran a construction article for a "Universal Battery Charger" intended for gel-cells. Basically, it uses a LM317 as a constant current source, and when the terminal voltage approaches full charge, it trips an SCR that brings the circuit back as a voltage regulator for a constant voltage trickle, or "float" charge. Klay-Corp in New York makes PC boards for the charger. I changed the layout to use it as a motorcycle battery charger (higher charge rate) and sent the artwork to Klay-Corp, so be sure to specify which version you want. BTW, you can find the complete motorcycle battery charger article in rec.motorcycles. Klay Corporation, Inc. 106 Mark Drive Syracuse, NY 13209-1808 (315) 635-5862 ---------------------------------------------------------------------- Kristopher Means Luck is the residue Van Nuys, California of design. (818) 343-5850 email: sometimes, not always. UUCP: {ames!elroy, <backbone>}!gryphon!pnet02!kris INET: kris@pnet02.gryphon.com
dkazdan@cwsys2..CWRU.Edu (David Kazdan) (12/06/89)
For lead-acid battery charging, Unitrode makes a chip that looks really good: the 3906 (2906, mil-spec). I have a couple but have not built the charger yet. It monitors charge current and voltage, keeps the charged battery going through a slight charge/discharge cycle, monitors ambient temperature internally to adjust its charging parameters, etc. The manufacturer's application notes are involved. There was a cookbook sort of article about it in QST for July (I think) 1987. --David
bill@videovax.tv.tek.com (William K. McFadden) (12/09/89)
In article <28220@amdcad.AMD.COM> phil@diablo.amd.com () writes: >I just found a source of 12 V 6 AH sealed no maintenance lead acid >batteries for $10 each. New? How can I get some? >What is an appropriate fast charge rate? Usually .2C max. for gel-cells, which is 1.2 A for a 6 AH battery. >Is it ok to trickle charge them in a burglar alarm application? Yes. You will want to use 2.35 V/cell, e.g., 14.1 volts. (Some people like 2.3 V/cell, giving 13.8 volts.) >What does the discharge curve look like? At 25 degrees C, the open circuit voltage is about 2.18 V when fully charged declining linearly to 1.98 V when 10% of capacity is left. >How should a fast charger know when to turn off? If you use a regulated supply for your charger, it will shut off by itself as the battery reaches full charge. A constant 2.35 V/cell charger will recharge a battery to 90% within 2 hours, and the battery can be left float charging at 2.35 V/cell indefinitely to maintain full charge. Extremely dead cells may go above the .2C rate if fed from a 2.35 V/cell charger. Going above this rate may cause excessive outgassing, which will eventually dry out the electrolyte. It is best, therefore, to limit charging current to .2C for gel-cells (which will result in a longer recharge time). Wet cells can be recharged at higher rates. >Do lead acids have the memory problems of NiCads? No. However, it is important to fully recharge lead-acid cells, with 2.35 V/cell optimum for batteries that are cycled once per week or less, and 2.45 V/cell for batteries that are cycled once per day. >Is it ok to leave them flat? No. The minimum voltage allowed during discharge is 1.6 V/cell, and lead-acid batteries should not be allowed to self-discharge below 1.8 V/cell. If allowed to discharge below 1.8 V/cell while in storage, the battery will take longer than normal to recharge, and the next discharge cycle cannot deliver the rated capacity. Subsequent cycles, however, will result in an increase in capacity to the rated capacity. If you let a lead-acid battery discharge completely to 0 volts, the electrolyte turns to water and is therefore not conductive. You can remedy this, however. Using a constant-current source, charge the cells at a .1C rate. This may require as much as 20 V/cell if the cell is extremely dead. When the voltage falls to about 2.5 V/cell, switch to a constant voltage, 2.35 V/cell charge until charging current levels off (e.g., reaches a minimum). >What is the best condition to store them in? Since they're sealed, humidity is not a concern, and gel-cells may be stored or used in any position. As with all batteries, the rate of self-discharge is a strong function of temperature. After 5 months of storage at 20 deg C, the typical battery will have 60% capacity remaining. At 40 deg C, however, the battery would be fully discharged after 5 months. >And out of curiousity, how do the corresponding NiCad systems work? Standard chargers normally charge at .1C for 14 hours, since this allows a very simple charging circuit -- batteries can be overcharged at .1C for extended periods without drastic reductions in life. Fast chargers usually sense temperature, starting as high as 4C, and tapering off as the cells get warm. I have also heard of fast chargers using temperature compensated voltage detection and detection of the voltage reduction that occurs after full charge has occurred. Nicad batteries in memory backup applications are usually continuously current trickle-charged at .002C to .1C. I can tell you more about nicads if you're interested. For a cheap charging circuit for lead-acid or nicad batteries, see LB35 in the National Semiconductor Linear Applications Handbook. Most of my information comes from "Summary of Battery Chemistries," Maxim 1988/89 Seminar Applications Book, parts of which were quoted above. Additional information about reviving dead gel-cells came from an article posted by Jan Steinman to sci.electronics on 9/3/87. -- Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077 UUCP: bill@videovax.Tek.com, {hplabs,uw-beaver,decvax}!tektronix!videovax!bill GTE: (503) 627-6920 "The biggest difference between developing a missle component and a toy is the 'cost constraint.'" -- John Anderson, Engineer, TI