9VDC battery as a backup battery
LoopyByteloose
Posts: 12,537
Can one simply put a 9VDC battery in parallel with a 9VDC supply line as a backup in case of a disconnect. Or does one have to consider using a blocking diode to prevent damage to the battery. Alternatively, can one use a NiMH rechargeable or will it become overcharged?
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Ain't gadetry a wonderful thing?
aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
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Ain't gadetry a wonderful thing?
aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
Comments
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24 bit LCD Breakout Board coming soon. $21.99 has backlight driver and touch sensitive decoder.
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·"If you build it, they will come."
I've not seen a 7.2v one for a few years. All the ones I have here are 8.4 or 9.6v. They certainly did exist however, and were a pain in the posterior!
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If you always do what you always did, you always get what you always got.
The circuit goes directly to a 5 volt regulator, so I don't think I have to supply a slightly higher than 9v supply in order to offset the diode voltage drop. The battery might simply discharge to 8.3-4 volts and then stabilize at that point.
I am a bit surprised by the range of 9volt rechargeables - 7.2v to 9.6.
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Ain't gadetry a wonderful thing?
aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
Post Edited (Loopy Byteloose) : 11/25/2009 9:23:34 AM GMT
·I would use a 317 in current mode set to··ONE 10 th of the amp hour of the battery plus what your Mic Controller plus what other·items ·that you are going to use and every so often discharge and re charge those battery they should work good for you for·years
In this set up you do not need to adj the voltage for the batteries the regulator will do that for at this low current·setting this not true at higher current please be aware of this fact
In this set up you are really running off the batteries that why you use ONE tenth of the battery amp hour rating so you do not over charge the batteries
For example· a battery· having a amp hour rating is 1000 mill amp so that mean 10 millamp charging rate
it will take about 20 hours or more to charge from a battery· that has a 1.0 volt reading at rest
I have run this test before and work very well
I hope this helps
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··Thanks for any·
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Sam
I was initially considering using a 9VDC wall wart with a 9V non-rechargeable battery and no blocking diode. I just figured the steady 9VDC would keep the battery from discharging and a balance would be reached. It certainly is the easiest to build.
Once I put in a blocking diode (even one with only 0.3v drop (geranium)), The battery is going to somewhat discharge against the other supply. I suppose letting the battery sit at 8.7volts indefinitely would work pretty well. The extra construction isn't much additional work.
On the other side, if I use a NiMH at 9.6VDC (that is what I have on hand), it will never fully take a charge unless I go to a higher voltage wall wart (say 12Volt as I suspect that is what I'd have to provide). Suddenly I am presented with providing some way to get back down to 9.6VDC and maybe current regulation alone will provide it. But I fear destruction from the heat of overcharging (even at a trickle charge) may eventually destroy the battery in less time than the first alternative. It is also getting into more of a part count.
As I have planned it, the device will be on the grid 24/7 except for power outages or some oddball interruptions. Then, the battery will provide standby power for a few hours; but the current drain is not very predictable because the board drives 8 rely coils that could be in any configuration from all off to all on.
For me, the Pro and Con is that the NiMH don't seem to last as long as the regular Alkaline if left in a simple waiting state (they loose their charge in weeks). Also, NiMH don't do very well with continuous tickle charge as heat will destroy them (and 1/10th of AH rating is NOT a trickle charge - it is their full rate charge). A trickle charge would have to be around 1/30th as anything less is considered harmful to the NiCad/NiMH battery.
So it seems there is a lot of extra engineering to accommodate a battery technology that isn't quite up to snuff. Lithium are great for quick recharges, Alkaline is great for lots of power, Lead Acid and Gel cell are great for perpetual trickle charges; but NiCad and NiMH seem to require more engineering and more replacement.
I've tried to use NiCad and NiMH in the past and they seem to almost never be as useful as the other choices. On the one hand, I keep running into voltages that don't quite fit other device's math; and on the other hand, charging is tricky -- while they don't quite adapt to perpetual trickle charge, they don't hold a charge long enough to be left lying around for months between occasional tasks.
I keep wondering what is going to work when the real power outage is a year or two down the road. I guess I could use a 6V Lead Gel cell and keep it on a steady trickle charge, but 1200mAH battery is about 8 times larger than the little 9V battery (the chassis starts to get very crowded).
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Ain't gadetry a wonderful thing?
aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
Post Edited (Loopy Byteloose) : 11/26/2009 3:44:04 PM GMT
I did some searching on the net and found patent circuits for a smoke alarm with a 9VDC battery that merely uses a blocking diode in series on a 12volt supply. Also I found a baby alarm with 9VDC backup without any diode on a 9volt supply. That seems to justify not doing much more as these are common consumer items.
I suppose I could try to locate some 8.4VDC rechargeable batteries and then have a near perfect fit for the voltage drop (they are apparently a Zinc battery), but I don't intend to recharge - jut have a very tiny compact standby. Computer motherboards use little 3.2V backup batteries for years that are not rechargeable batteries. So it appears to be sound to just keep it simple rather than have a charger circuit or a special 9.6 volt supply.
Kwinn,
I can't quite follow your last post. Thanks anyway.
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Ain't gadetry a wonderful thing?
aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
The simplest fix is to have the battery voltage minus the diode drops be less than Vin (see diagram). If the battery voltage is 9V and the silicon diodes drop 0.7V each then as long as Vin is more than 7.6V (9-1.4) no current will flow in or out of the battery. Diodes are cheap so if you need 3 or 4 in series to get the voltage out from the battery low enough why not use them.
It now makes sense. I had my math backwards (thinking I had to add voltage to Vin to compensate for the voltage drop). Since I am not trying to recharge and the regulator requires only 7volts for 5volt output, I just need to block the 9volt battery from discharging. It would seem that one diode would be enough and would allow me to use more of the battery's capacity when it is put into use. Also, a low drop out regulator would provide another power boost. Many of the alkaline batteries in the stores are currently dated to use before 2016 - so I suspect the shelf life is quite adequate.
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Ain't gadetry a wonderful thing?
aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
Post Edited (Loopy Byteloose) : 11/28/2009 10:28:14 AM GMT
1) 9v have very low amp hour ratings usually only 300-700mAh or so.
2) germanium diodes have much hier leakage current then silicone.
3) wallwarts are netoriusly not at the voltage they say they are at.
4) linear regulators are ineficient and will drain batery faster
How to deal with these things:
1) use 4AA bateries to give you 6V.
2) use silicone diode
3) use as high a voltage wall wart as you can without over heating the regulator and use a large cap in parallel with it.
4) use buck/boost switching regulator. will give you 80-90% efficiency instead of 20%.
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24 bit LCD Breakout Board coming soon. $21.99 has backlight driver and touch sensitive decoder.
1)I would use 4AA batteries - if I had room for them. 700mAH is likely enough for now.
2) I can easily adapt to silcone diodes and the greater voltage drop.
3) Older wall warts are very bad at outputting exact voltages(I have some and they usually run high), but the newer wall warts are often spot on (due to better designs).
4) Linear regulator inefficiency in this context are a moot point as the battery will most often be in a full charge standby condition - AC mains will drive the regulator. The board came with a linear 6 volt regulator driving an 74LS373 and 8 relays.
5) I suppose I come provide 6 volts on the other side of the regulator, but again I have to consider room for 4AA batteries or some other set up.
So, where am I going with this?
Either I'll stay with the very wimpy 9V design or add an external 8-9V gel cell with ample AHs and a trickel charge.
It may be that I just expected to fit everything into too small a package. Every time I get involved with either AAA batteries or 9V batteries, they seem to be just too darned tiny. But do wonder how devices like smoke alarms manage to exploit them as standby power over a period of several years.
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Ain't gadetry a wonderful thing?
aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan