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#21
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Battery question - two 7.2 V packs to get 14.4 V
Werehatrack wrote: (clip) If a higher-current charger was obtained, the two would almost certainly not charge at the same rate...thus one would begetting too much current, and one too little, to charge properly.(clip)^^^^^^^^^^^^^^ Jasper wrote: And *that* is the real problem. ^^^^^^^^^^^^^^ It may not be as big a problem as it seems. If the two batteries are reasonably well matched, and they are connected in parallel to a charger which provides a voltage a little higher than the batteries, each will draw some current, in accordance with their internal resistance and voltage difference. The one which draws more current will charge more rapidly, so it will taper off more rapidly, and they will eventually both reach a fully charged state. This is a situation where the unbalance is self-correcting. |
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#22
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Battery question - two 7.2 V packs to get 14.4 V
On Sun, 27 Nov 2005 02:48:15 GMT, "Leo Lichtman"
wrote: Werehatrack wrote: (clip) If a higher-current charger was obtained, the two would almost certainly not charge at the same rate...thus one would begetting too much current, and one too little, to charge properly.(clip)^^^^^^^^^^^^^^ Jasper wrote: And *that* is the real problem. ^^^^^^^^^^^^^^ It may not be as big a problem as it seems. If the two batteries are reasonably well matched, and they are connected in parallel to a charger which provides a voltage a little higher than the batteries, each will draw some current, in accordance with their internal resistance and voltage difference. The one which draws more current will charge more rapidly, so it will taper off more rapidly, and they will eventually both reach a fully charged state. This is a situation where the unbalance is self-correcting. Except it's not. It'll kill your batteries in fairly short order. I'm not entirely sure what the mechanism is, but that's both received wisdom and my experience. Jasper |
#23
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Battery question - two 7.2 V packs to get 14.4 V
Jasper Janssen wrote: "Leo Lichtman" wrote: It may not be as big a problem as it seems. If the two batteries are reasonably well matched, and they are connected in parallel to a charger which provides a voltage a little higher than the batteries, each will draw some current, in accordance with their internal resistance and voltage difference. The one which draws more current will charge more rapidly, so it will taper off more rapidly, and they will eventually both reach a fully charged state. This is a situation where the unbalance is self-correcting. Except it's not. It'll kill your batteries in fairly short order. I'm not entirely sure what the mechanism is, but that's both received wisdom and my experience. I think at least one issue is that the battery which is in worse shape will usually have higher internal resistance, so more current will flow through the better battery. However, I don't believe that its internal resistance increases much (if at all) as it reaches a charged state, so the two batteries don't equilibrate. In the meantime, you have the good battery which is capable of delivering 7.2 volts (or whatever) and the worse battery which is at a slightly lower charge and voltage. But these two are tied together in parallel, so they're pinned to the same voltage and you may actually be discharging the good battery through the worse one. Lord knows what dV/dt a smart charger is measuring off this setup. It seems like a false economy to me. |
#24
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Battery question - two 7.2 V packs to get 14.4 V
Jasper Janssen wrote:
Except it's not. It'll kill your batteries in fairly short order. I'm not entirely sure what the mechanism is, but that's both received wisdom and my experience. The reason is the voltage characteristics of NiMH cells as they charge. The voltage increases slowly as the cell charges, and then drops a little as full charge is reached. In a parallel charge situation, the one to reach full charge first then gets all the charging current (plus some more from its undercharged, higher-voltage neighbour). John |
#25
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Battery question - two 7.2 V packs to get 14.4 V
"John Henderson" wrote: (clip) In a parallel charge situation, the one to reach full charge first then gets all the charging current (plus some more from its undercharged, higher-voltage neighbour). ^^^^^^^^^^^^^^^^^ I can see no way that one battery can supply current to the other while they are both hooked to an outside source that exceeds their voltage. How can the higher voltage battery supply current to the lower one, while at the same time drawing current from the charger? Current can't flow both ways through the battery at the same time. If you left the two batteries hooked together in parallel and disconneced them from the charger, this would be possible, but that's not what we're talking about here, and it would be a dumb thing to do. |
#26
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Battery question - two 7.2 V packs to get 14.4 V
Leo Lichtman wrote:
I can see no way that one battery can supply current to the other while they are both hooked to an outside source that exceeds their voltage. How can the higher voltage battery supply current to the lower one, while at the same time drawing current from the charger? Current can't flow both ways through the battery at the same time. If you left the two batteries hooked together in parallel and disconneced them from the charger, this would be possible, but that's not what we're talking about here, and it would be a dumb thing to do. How could the supply voltage exceed that of the lowest-voltage battery? In any unimpeded current-limited situation, it's (lowest) battery voltage which sets supply voltage. John |
#27
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Battery question - two 7.2 V packs to get 14.4 V
Sharp wrote:
Hello Can anyone tell me can I connect two 7.2 V Ni-MH packs in series to get 14.4V, use it for powering 20 W halogen lamp, and after power runs down disassemble it and charge 7.2 V packs separately, first one and then another. Is it possible to do this and not damage the batteries? I own smart battery charger that can charge only 10 cells (MW 7168 omni) and I need 14.4 V battery. This is indeed a common problem, since there are only a few NiMH chargers that can charge 12 or more cells in series at a time. What you want to do is to have a two pin connector across the + and - of each pack. These connectors are for charging. The connector from the + of one pack to the - of the other pack, goes across all six batteries, and is what connects to the light. You'll have to manually move the charger from one pack to the next. There are ways of automating the process of charging two packs, sequentially, using a timer and relays. See "http://nordicgroup.us/s78/chargers.html" and look for the section entitled "Unattended Sequential Charging of Multiple NiMH Battery Packs with a Single Charger." Note that the Maha MH-C777PlusII "http://thomas-distributing.com/mh-c777plus.htm" will charge 12 cells in series. This would almost certainly be a better solution than futzing with connectors, timers, and relays. |
#29
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Battery question - two 7.2 V packs to get 14.4 V
SMS wrote: Sharp wrote: Hello Can anyone tell me can I connect two 7.2 V Ni-MH packs in series to get 14.4V, use it for powering 20 W halogen lamp, and after power runs down disassemble it and charge 7.2 V packs separately, first one and then another. Is it possible to do this and not damage the batteries? I own smart battery charger that can charge only 10 cells (MW 7168 omni) and I need 14.4 V battery. This is indeed a common problem, since there are only a few NiMH chargers that can charge 12 or more cells in series at a time. What you want to do is to have a two pin connector across the + and - of each pack. These connectors are for charging. The connector from the + of one pack to the - of the other pack, goes across all six batteries, and is what connects to the light. You'll have to manually move the charger from one pack to the next. There are ways of automating the process of charging two packs, sequentially, using a timer and relays. See "http://nordicgroup.us/s78/chargers.html" and look for the section entitled "Unattended Sequential Charging of Multiple NiMH Battery Packs with a Single Charger." Note that the Maha MH-C777PlusII "http://thomas-distributing.com/mh-c777plus.htm" will charge 12 cells in series. This would almost certainly be a better solution than futzing with connectors, timers, and relays. ISTM that spending $50 to $100 on a charger, and/or cobbling something together from timers and relays and connectors doesn't make sense unless your hobby is building electronic toys, not bicycling. I've played around a bit with electronics, but it's not really my thing. I certainly never subscribed to "Popular Electronic Stuff To Build In Your Basement." So I prefer to use lights that simply _work_. - Frank Krygowski |
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