I do a lot of voluntary work for a local youth amateur theatre group.

One major problem we have is the cost of purchasing 9 volt (PP3) batteies for the 6 wireless microphones we use. On average a single show, including rehearsals, will use around £100 worth of batteries. Tried the easy option with rechargeables but at only 8.4 volts they simply don't work.

My idea is to use either a Sealed Lead Acid battery (12 volt) and a 9 volt fixed voltage regulator (7809) or a 9.6 volt racing battery pack.
I am very tempted to use the SLA battery and regulator but wonder if the 9.6 volt pack would be likely to cause any damage to the 9 volt system.

Either way, an outlay of £100 would provide virtually free power for a few years at least.We are currently spending £400 a year on batteries.

Any other ideas would be more than welcome.

Bernard

 

I do a lot of voluntary work for a local youth amateur theatre group.

That is commendable. Megga dittos.

One major problem we have is the cost of purchasing 9 volt (PP3) batteies for the 6 wireless microphones we use. On average a single show, including rehearsals, will use around £100 worth of batteries. Tried the easy option with rechargeables but at only 8.4 volts they simply don't work.

Why does your charger only charge them to 8.4 volts? It seems to me it might be the problem?

My idea is to use either a Sealed Lead Acid battery (12 volt) and a 9 volt fixed voltage regulator (7809) or a 9.6 volt racing battery pack.

And the actors and actresses are going to wear this 12 volt battery and regulator on their persons? Where? LOL. L with you, not at you.

I am very tempted to use the SLA battery and regulator but wonder if the 9.6 volt pack would be likely to cause any damage to the 9 volt system.

I would try it. How much can you lose?

Either way, an outlay of £100 would provide virtually free power for a few years at least.We are currently spending £400 a year on batteries.

So you'll be a hero! Best wishes!!!

 

Take a look at CR123 batteries. They are slightly larger in diameter than a AA battery, and 2/3 as long.

Put three in series for a pack that is 4" long, or next to each other (up/down/up) for a pack that it 1.3" tall, 3/4" thick, and 2.25" wide. 1.5AH capacity with non rechargeable, 900mAH with rechargeable - Though voltage on rechargeable Li-Ion varies between 3.2 and 4.2 V! Non-Rechargeable have a relatively flat discharge curve, high capacity, very long life/low self discharge.

Experiment, find average drain of the wireless mic, or run one from 3 123's until range suffers. Typically, 123s will last over 5x as long as a 9V alkaline, and handle high current loads as well.

The price on them in bulk is cheap due to their "perfect fit" with LED lights from Surefire, Streamlight, Pentagon, and other manufactuers. Essentially, pocket sized sunlight, insanely bright.

Link, note different voltage on rechargeable!

 

Why does your charger only charge them to 8.4 volts? It seems to me it might be the problem?

You are lucky to find 8.4V rechargeable batteries. Most have six 1.2V cells for only 7.2V like a slightly used "9V" alkaline battery.
But their capacity is very low.

 

Quick question: Are the batteries tested, or are new ones always used?

I know the local theater and every band I've met simply puts new batteries of all sizes in everything battery operated before a show, sometimes even before rehearsals. The audiences do not tolerate failure very well.

A great invention would be a "RAIB" (Redundant Array of Independent Batteries). Two 9V batteries, only one active, sub-millisecond switchover time after a certain voltage level. Then all batteries would be used for their entire useful life, and "battery paranoia" would go down a bit. They make Ultralife 9V/PP3 ~1AH batteries, using 3 lithium cells, at a hefty priceof $5+ each, yet I've seen those used for a single 3 hour show and thrown away with alkalines (400mAH - 500mAH).

 

Thanks for the info guys.
The C123 looks interesting.
Thgink I'll give the C123 layout a go and also the SLA.
A SLA 800mAh battery is only the size of a cigarrette packet and a 7809 regulator is small and may not even require a heatsink.
I'll let you know the outcome.
No to find C123's in the UK !!
Thanks again
Bernard

 

Have you thought of a boost regulator IC? Some have excellent efficiency, and would end up as an overall smaller package than one 9V battery with a CR123.

A linear regulator wastes a lot of power when current is drawn. Switching regulators draw low current from the battery when there isn't a load.