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Old 8th Aug 2011, 11:11 am   #1
mark pirate
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Default 90V HT Battery.

I have been given a quantity of ni-mh rechargeable batterys, mostly 200ma PP3's.
After giving a few a charge, they seem to be fine, so i have decided to put them to good use by making an ht battery, this brings up a couple of problems, 10 batteries will give me 84 volts, 11 will be 92.4v, so either side of the 90v required.

The second problem is charging them, i have three chargers that take two PP3's each, so i can charge six at a time, but this is going to be a pain, but i cant think of an alternative

I want to construct a simple wooden box with a hinged lid equipped with copper strips to connect all batteries in series, rather than using a load of battery clips, as i think this will be a neat solution & will allow easy access to the batterys.

Any thoughts?

Mark
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Old 8th Aug 2011, 11:30 am   #2
slderiron
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Default Re: 90V ht battery

Hello Mark, regards the 10 pp3's, I had one of these made by a forum member which is enclosed in a period looking Ever Ready mock-up 90v B126. It works perfectly well on 84v, powered up the radio no problem. Probably safer to operate just under than just over 90v.
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Old 8th Aug 2011, 11:50 am   #3
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Default Re: 90V ht battery

90V batteries are not actually 90v.

If they are 60 x 1.5V Carbon Zinc cells they will vary from a bit more than 90V when fresh to about 66V when totally flat. Alkaline would be 98V "fresh" dropping to about 68V when "flat".

If you discharge the NiMH PP3 too much, one cell in one PP3 will be flat before the rest and then reverse charged and destroyed.

The normal "end point" of a single NiMH is about 0.9V. If stacked in a battery, 1V per cell is a safer end point. A PP3 with 6 x NiMH cells would be about 8.7V to 9V fresh, about 8.1V for initial discharge typically falling to 7.2V and then 5.4 to 6V when "flat".


1.5V filament Battery Valve radios designed with Nominal 45V, 67V and 90V HT batteries typically and data sheets for later Dx96 series show 67V and 90V characteristics. As batteries go flat performance degrades due to lower gain, though between 75V and 100V likely not much difference with Pentodes.

Download datasheet for LT1073. Look at 90V supply for the pulse generator.

You can plug the PP3s into each other and use one clip cut in half at each end.

10 or 11 PP3s will work. Using Alkaline I'd use 10 x PP3 and using NiMH I'd use 12 actually.

But the biggest issue with the NiMH is not over discharging them and damaging one cell in one pack.

I'm designing a battery radio with 45V on Receive and 45V + 135V nominal on TX. I was going to use loads of NiMH AA cells to avoid "modern" electronics. But I think I will make two sets of power packs, one using Alkaline cells for 1.2V, 45V, 135V and one using 8 x C or 8 x D NiMH and LT1073 based inverters, a boost for 45V, a boost with external switch for 150V and a buck for 1.2V filament. Then the pack can charge from 12V to 14V external power inc car Cigar socket.

There is no simple protection for 10 to 12 x PP3 rechargeable (or 60 to 72 AA cells) in series without additional electronics like a micro-controller with 6 analogue inputs. (You can judge 2 x PP3 in series safely).
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Old 8th Aug 2011, 11:52 am   #4
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Default Re: 90V ht battery

In summary:
10 x alkaline PP3
12 x NiMH PP3, but take care to avoid over discharging.

For example the handheld CBs I have use AA cells to give a "nominal" 12V There are two "dummy" cells to fit when using Alkaline instead of the NiCd (NiCd are slightly lower voltage than NiMH, especially at first part of curve, about 1.25V vs 1.32V).

Last edited by neon indicator; 8th Aug 2011 at 12:02 pm.
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Old 8th Aug 2011, 12:06 pm   #5
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Default Re: 90V ht battery

The simple and lethal way to charge the whole battery is to apply mains via a suitable resistor and a diode or bridge. Keep the current well down to avoid overcharge problems. Probably something like C/50 or 4mA so the charge time will be correspondingly long. A 33K resistor should be about right and will not dissipate much more than a watt.

Reitererating that this a nasty and dangerous way to do the job but it should work. I think NimH will tolerate C/50 indefinitely but ought to check data sheets before trying.
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Old 8th Aug 2011, 1:21 pm   #6
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Default Re: 90V ht battery

Re being slightly on either side of 90v this wont make the slightest difference as you are only talking a few percent either side, well within manufacturers tolerance limits.

As for charging them well I think I would go for either a half wave diode via a current limiting resistor around 3200 ohms but you would need an 8 watt resistor.

Alternatively why not use a capacitor instead and save energy, if my calculations are right a 1uf capacitor would be near the mark but it must be non polarised, definately not an electrolytic.

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Old 8th Aug 2011, 4:57 pm   #7
mark pirate
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Default Re: 90V ht battery

Quote:
You can plug the PP3s into each other and use one clip cut in half at each end.
I am currently doing just that!
I have 10 fully charged 8.4v PP3's connected to my battered old sky queen, i have the set at an average volume & am getting 79.8 volts on load, the set is performing fine at this voltage, i will run the set for a few more hours while monitoring the voltage.

I was told that the batteries are unused, but individual testing with my Mallory professional battery tester is showing the uniross branded batterys are giving a higher output than the maplin & rs branded batteries, so i am using these.

Quote:
If you discharge the NiMH PP3 too much, one cell in one PP3 will be flat before the rest and then reverse charged and destroyed.
I will run all 10 through the tester tomorrow to see if any of them have discharged more than the others, what would be a good point voltage wise to recharge them?

Quote:
The simple and lethal way to charge the whole battery is to apply mains via a suitable resistor and a diode or bridge.
As for charging, i think i will stick with the standard ni-mh chargers i already have!

The main reason for this ht battery project, is to allow occasional use of my four sky queens as they were intended to be, i.e. portable

I have thought about stripping out some original batterys i have (see photo) & fitting 10 poundshop specials, but i cant bring myself to destroy them!
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Old 8th Aug 2011, 5:09 pm   #8
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Default Re: 90V ht battery

Quote:
Originally Posted by mark pirate View Post
individual testing with my Mallory professional battery tester is showing the uniross branded batterys are giving a higher output than the maplin & rs branded batteries
I have come across ni-cad PP3's of different manufacture, some of which use six cells internally (7.2V nominal voltage therefore) and some which use seven (giving 8.4V). You could well find this is the case.

As for reverse-charging, it is easy to connect a 1N4003 diode in inverse-parallel across each battery, to prevent this. (Although not if you have clipped them all together!)
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Old 8th Aug 2011, 5:34 pm   #9
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Default Re: 90V ht battery

The 1N4003 across each pack (even a 1N4148 may do) will protect a pack, but not one cell in a pack.

The minimum per pack voltage for NiMH (NiCd are a little different) depends on if a 6, 7 or even 8 cell pack. Most NiCd is 0.9V per cell minimum, NiMH is normally recommend 1V if not a standalone cell, i.e.

6 cell PP3 = 6V min (About 8.1V no load and reasonably fresh, but not on charge, 7.2V @ 50% charge)
7 cell PP3 = 7V min (About 9.45 no load, fully charged, 8.4V approx @ 50% charge)

Note very high capacity NiMH usually self discharge in 2 to 4 weeks unless eneloop type, NiCd has longer life and lower self discharge, but about 1/2 or even 1/3rd capacity.

You can get ready to colour print images online of the 90V packs and make replica boxes.

It could be that 200mAH are 6 cell and 180mAH are 7 cell. Or not.

Actual NiCd are rare now, most are NiMH. There seem to be 3 kinds of NiMH
1) Ordinary: AA cells are 1800mAH to 2200mA
2) High capacity 2500mA to 3000mA (probably really only up to 2700mA). Most of these have rapid self discharge and many models must not be trickle charged any faster than self discharge, i.e. can only be "fast charged", or trickle charged for fixed time. and then maintained at very low trickle.
3) Eneloop type. Medium to high capacity, very low self discharge. No continuous trickle charge allowed.
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Old 9th Aug 2011, 10:35 pm   #10
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Default Re: 90V ht battery

Quote:
Originally Posted by mark pirate View Post

I have thought about stripping out some original batterys i have (see photo) & fitting 10 poundshop specials, but i cant bring myself to destroy them!
I wouldn't worry about 'destroying' them, you're still preserving something, making it functional at the same time.

I've got a combined LT+HT battery in a 'Sky Baby', i've gutted that a long time ago, i'd rather have something original looking and functional than original looking and as much use as a chocolate teapot.

A 90v (ish) original looking rechargeable battery is something special indeed.

Dave.
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