Quickie with rechargable batteries

[Natural Sound]

The company I work for uses an 8.4v 1.2aH battery backup in some of its equipment. These are changed every year but still have plenty of life in them when they go to the recycle bin. Basically they are 7 C-Cell NiMH cells in series. I've been playing around with these to run some DC electronics around the house and in the back yard. I charge these packs at a C/10 rate or 120mA for about 15 hours. This seems to work out pretty well.

I'd like to use these battery packs for my quickie B+ supply. My question is this. would it be better to use four of these for a B+ of 33.6v or five of them for a B+ of 42v?    

 

[Paul Joppa]

Standard alkaline cells start out a bit over 1.5v, and are considered dead by the time they drop to 1.0 volts. So a "9-volt" battery will run 6 to 9 volts over its working lifetime. NIMH and NiCd cells have a pretty flat discharge voltage of 1.2 volts, so even a 7.2v battery would be enough.

That said, you will get more headroom with more voltage. The circuit will behave reasonably well with up to 67 volts, which is the maximum rated screen voltage for the 3S4.

For what it's worth, I originally chose to use four batteries because they often come in packs of eight at good prices - you can get two complete sets that way. A second advantage is that the battery lifetimes are matched, about 200 hours for the D-cells and for the 9-v batteries.

 

[Natural Sound]

Excellent, thanks Paul! I'll run it at 42 volts then.

 

[minodev]

The Tenergy TN141 "Centura" <http://www.amazon.com/Tenergy-Charger-Self-discharge-Rechargeable-Batteries/dp/B00461DNTO/ref=sr_1_6?ie=UTF8&qid=1347286055&sr=8-6&keywords=tenergy+9v> rechargeable NiMH 9v set seems very reasonable and a good option for the Quickie. They're only 200mAh, but they're "low self-discharge" which seems like it would match well with the low draw of the Quickie.

There's also a 250mAh version <http://www.amazon.com/Tenergy-Charger-250mah-Rechargeable-Batteries/dp/B0045WGWCE/ref=sr_1_8?ie=UTF8&qid=1347286055&sr=8-8&keywords=tenergy+9v>, but they're not "low self-discharge." Rather, they claim to be good for RC cars and such, where I guess the plan is to burn through your batteries in a single sitting.

Anyone have any experience with these? I might pull the trigger on a TN141 set and report back.

 

[4krow]

I am anxious to get my Quickie done now, in order to see what the Li-ion 9v will do. They measured 42v when tested in series, so that is a plus. How they will discharge I do not know. I was just thinking of the extended life that they seem to be capable of. Though I don't recall their milli-amp/hr rating, I do remember that it was higher than the other batteries that I considered.

 

[jrebman]

Minodev,

I'm going to be building my quickie with rechargeables for both the 1.5v D cells and the 9 volt ones.  The 200 mA rating is somewhat lower than a standard alkaline, so I bought 4 more 9 v cell holders and plan to run to independent B+ supplies using 4 of these cells per channel.  That should make the battery life for both the D cells and the 9v cells about the same.  I also have a charger that will charge from 1 to 10 9 v cells at a time, and another that will charge 2 D or C cells at a time, so I will be able to charge the whole supply at once.

-- Jim

 

[tyerkey]

I am really interested in an AC power supply for my quickie.  I read an earlier post that one might be in the works.  If so, when?  If not, does anyone have a DIY solution?

 

[Paul Joppa]

I can't say when a prototype PSU might get built, probably a few months - most effort is still going into the 300B preamp right now. At present I am envisioning a largish PC board that fits under the chassis, below the battery area, with an AC wallwart to bring power in. I have a preliminary design in mind but it's just sketches so far.

A big question is whether I can make it optionally function as a trickle charger for NiMH batteries, and if so how many switching options are needed...

 

[Paul Joppa]

OK, did a little research. No battery charging. Charging NiCd and NiMH is a rats nest! The most practical is a trickle charge, generally frowned on by the cognoscenti - and it would double or triple the PSU demands, and is highly dependent on using the right size and composition of cells - using a low-capacity D-cell (the ones that are usually available and cheapest, with AA or C cells inside) would result in possibly violent destruction of the cell. I'm not going to go there.

The good chargers are smart enough to need a microprocessor on board, plus voltage and current A/D converters and clocks. That's a whole different technology - not a fun one for me. For those interested, there are some decent articles on Wikipedia.

 

[autoformer]

I'd like to try running a Quickie off a single 36 or 48v LiFe pack.  Those voltages are ok for the B+, but what would be the best (i.e., cleanest, quietest, most efficient) way to reduce voltage to 1.5v for the filaments?  Would a simple voltage divider do, or is there a better way?

 

[jimiclow]

I'll be building a Crack once I finish the SEX (still on order). My plan would be 2 x 6S1P 26650 A123 (39.6 volts 2300mah). This way, I can charge them in parallel up to 20 amps with my R/C charger. It will be fully charged in about 10 mins. These batteries are robust and have cycle life of 2,000. I think I'll stick with D NiMh that comes in 10-11Ah for the filaments. Charging will still be with the R/C charger at 1C.

 

[Paul Birkeland]

would be the best (i.e., cleanest, quietest, most efficient) way to reduce voltage to 1.5v for the filaments?  Would a simple voltage divider do, or is there a better way?

It's not quite that simple, you either need two separate floating supplies for the filaments, or you can combine them together and run the circuit in fixed bias.  Fixed bias could be accomplished with an input capacitor and a battery in series with a resistor to bias the grid (this battery would last for years, the current draw would essentially be zero).

 

[Paul Joppa]

The problem with fixed bias is that it does not adapt to changing high voltage as the batteries drain. Similarly, the problem with a common filament supply is that it does not adapt to mismatched tubes. Both problems have solutions of course, but they bring increasing complexity and thus their own problems.

 

[autoformer]

Could you use some sort of regulator to keep the filament voltage constant as the batteries drain?  A LiFe pack has a very flat discharge curve, so voltage drop as the pack drains will likely not be too much of a problem.  I'd love to figure out how to power a Quickie off a single 12S LiFe pack and, as jimclow says, charge it with an R/C charger.

 

[Grainger49]

If the LiFe pack is only available in 36 or 48 volt increments you will have to waste a lot of voltage in the voltage divider to bring it down to the 1.5V for the heaters.  You could do it with a second LiFe pack.  If you were thinking of using the same one as you would use for B+ then you have to float the takeoff for the heaters the voltage of the cathodes.  If you look at the schematic the heater's 1.5V is from the top of the cathode resistor and bypass capacitor to the outer leads of the heater.  Center tap of the heaters is the negative side of the heater supply. 

So to sum up, the heater supply negative lead has to be above the common for the B+.