Another Cheaper, Automatically "Safe" Charger.

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To: Schetgen, Bob, KU7G
Sub: Another Cheaper, Automatically "Safe" Charger.

I'd like to clarify a few things that it seems Bob Lewis got mixed up. If Bob managed it then I guess it's my fault and I would assume others may also have been confused.

First, Bob states, "An additional potential problem is that Phil's charger depends totally on the battery to provide the filtering ..." This is not true. The Wall Wart that was used was a 300mA 12DC filtered Wall wart, not an AC version. This is clearly explained in my web site article, but not in this article, sorry about that. It was implied in the statement, “I was originally charging gel-cells directly with a 12-V, 300-mA wall wart I had around.” I assumed everyone would know that you don’t use a 12V-AC wall wart to charge a DC battery directly, my mistake.

Second, Bob states that I am, "...depending on the battery to provide the filtering necessary to remove the ripple from the half-wave rectifier." This is also not true. The external diode is not there to rectify the wall wart voltage to DC since that is done with a full wave rectifier inside the wall wart. It is there to insure that the battery does not discharge through the limiting resistor.

I operated my 100W HF rig from this arrangement for years with no problem. No ripple was noticed on any of the nets I checked into either on CW or SSB, unless the battery voltage went well below the charger voltage, which it would do when I operated for extended times with multiple rigs going as I did during SETs. Then it's time to switch batteries.

Third, the limiting resistor is selected to insure that the charging voltage never exceeds the battery specification for trickle charging, i.e. for keeping it "topped off." The interesting thing is that if the battery is discharged, most of the current will go into the battery and not through the resistor but as the battery becomes fully charged more current goes through the resistor and less to the battery. It is "self" limiting, as long as you've selected the resistor value correctly.

Forth, while Bob had it correct, "Everything is unregulated" this is not the disadvantage Bob seems to think it is. He seems to be worried about "If the line voltage varies by 10%, for example..." Does Bob live where his power company gives 10% more voltage often and for long periods? I don't and I've lived in four states from California to New York, and visited many more, and so far I've never had this problem. If anything the power company voltage can be low for extended periods of time so this kind of charger won't completely charge the battery or keep it completely topped off until the voltage returns to normal. I have not seen even this as detrimental, since it too is not usually a normal condition either.

Fifth, Bob states, "I disagree with Phil's statements: (1) It does not hurt to leave an SLA battery sitting around for a year or more without any maintenance." I believe that I stated, "I have found that you can charge a good gel-cell once a year and leave it on the shelf." and "You do not NEED to keep them topped off." I left a fully charged battery around for a year or more and then used it. I still claim that this is OK and that the batteries we have today, while designed to accept continual topping off, do not absolutely have to be continually topped off to be in good shape. I'm not talking about being "fully charged," just in "good shape", after sitting around for a year. If I remember correctly the self discharge rate of this kind of battery is 1%, or less, per month. This means we'll still have 88% of full charge, or better, at the end of a year. Not perfect, but I'd call that in "good shape" wouldn't you?

In one respect Bob and I agree here and that is if the battery is put on the shelf when it is fully, or mostly, discharged. I believe then a sulfate film is produced either on the plates or between the connections? In any event, either will act as an insulator and make it impossible to recharge the battery. Perhaps Bob has a warning about that and can give us numbers, as in, "when the battery is down by 25% of full charge sulfate starts to form?" I don't know when it will start other than close to full discharge. So, I also advise never to leave an SLA battery sitting around in a mostly discharged state.

Sixth, I believe I stated "...undercharging problems are probably not the cause of most premature battery failures" and not as Bob said, "... Phil's statements...(2) It does not impact long-term battery service life to continually undercharge an SLA battery." I was comparing the probabilities of under-charging vs over-charging doing damage to a battery. I still maintain that undercharging a battery is probably, I'm talking probabilities here not certainties, not the cause of most, again MOST, premature battery failures. I don't deny that there is some possibility that this condition if continually used may cause premature failure, it's just no where near as probable as even an occasional overcharging.

I won't bother trying to go over that "forcing" issue, I will simply say I was trying to make a point and get some clarification not maintain that Bob's charge was forcing current into the battery. However, if his charger senses a voltage drop and then responds to it by upping his charge voltage I guess in a way this does "force" more current into the battery, doesn't it? After all by Bob's own words the load has increased, the recharger responds with a higher voltage and so if I = E/R we have E increasing and I guess this means that I must increase if R is the same (which, when you stop transmitting R - the load - will be.)

The bottom line is then,

1. Bob's charger is better than mine. His charges at a full 500mA till it is fully charged.

2. My charger uses a 300mA 12V(DC) wall wart so at best his will recharge the battery 67% faster than mine, but then if you want 500mA you can simply use that 500mA 12V (DC) wall wart you have in your junk box instead.

3. Bob's charger automatically reduces the voltage to that needed to replace the self discharge of the battery.

4. Selecting the resistor and using the position between the diode and resistor does the same thing, but you do have to hand move the clip from the faster charge location to this maintenance position.

5. Bob maintains that moving the power clip is an onerous task.

6. I don't think so, since if the battery was deeply discharged and it was even just a 7 AHr battery you could simply use the fast charge location over night and switch it in the morning. For larger batteries you'll need to check it before you go to work and before you go to sleep to see if it's close enough to full charge to put the clip on the maintenance location. I guess that could be onerous for some.

7. Bob's charger is much more expensive than mine.

8. His limits the charge voltage to 13.8 when fully charged using ICs. Kind of expensive.

9. Mine limits charge voltage to 13.6 - 13.8 using a diode resistor current splitter. Real cheap.

10. His charger is voltage regulated, independent of the AC line voltage, to normal limits.

11. Mine limits voltage through the wall wart transformer, full wave bridge, cap, and the external diode-resistor current splitter. So, if you live someplace where your AC line voltage is more often than not above 120 volts AC by 10%, you simply take that into account when you select the correct resistor value and be done with it. If on the other hand you live in a place where the power company hardly ever exceeds their designed voltage for extended periods of time, then again, select the resistor to limit the voltage at full charge to 13.6 - 13.8 for these conditions and be done with it.

12. Bob seems to imply that you MUST keep these kinds of batteries topped off to maintain their life.

13. I maintain that you can do this with todays batteries, but that it is not mandatory. In the manufacturer's specs I believe they are talking about, wanting to maintain the battery at full charge all the time, then your "...losses...must be replaced at the rate the battery self discharges...", I don't disagree with that, seems self evident to me. But if you're not worried about keeping it topped off all the time, you can charge it, shelf it, and check it every few to 12 months, at your discretion, and top it off when you feel it needs it.

Oh, and last, you may not be able to get directly to my trickle charger article with the URL that was provided in the last article. If that is the case go to: and then do a search for "charger" and click on the link to the article. I tend to use JavaScript techniques to keep people from using my pages as part of their sites as if they had written them or asked permission to do so when they have not.

Phil Karras, KE3FL


Subject: RE: Another Cheaper, Automatically "Safe" Charger.
Date: Wed, 27 Nov 2002 10:13:05 -0500
From: "Schetgen, Bob, KU7G"
To: "Philip Karras"

Thanks Phil. I have it. If there’s room, the Feb column will include a note that Zack gave me when he reviewed the Dec column:

By Zack Lau, W1VT, ARRL Lab;

Page 8 of the Web document Bob Lewis cites says "one recharge per year is sufficient to maintain the original capacity of a battery not in use." The Rechargeable Batteries Application Handbook by the Technical Marketing Staff of Gates Energy Products speaks on the topic as well (pp 203-204).

"The key to successful storage of sealed-lead batteries is maintaining a minimum level of charge in the cell or battery. As long as the open-circuit voltage remains above this cutoff, the battery does not experience any irreversible changes that affect capacity or life."

Notice that the Gates book does not contradict your experience. It simply indicates that your experiment probably kept the battery above the minimum charge level. Later in the Gates book, it specifies 1.8 V per cell as the minimum, with a storage life of near 36 months at room temperature (Figure 4-45 and paragraph on p 205).


"Schetgen, Bob, KU7G" wrote:

I think we've spent enough space on the discussion. Readers of a "quick and dirty" inclination will likely use your method, "straight laced" types will go for the complex charger. Thanks anyway.

73, Bob, KU7G

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