Battery Modification Question

[Ham]

There are several metal detectors on the market that take 8 AA penlight cells. I was considering making a mod where i could substitute a 12vdc gel cell to replace the AA batteries. I don't mind wearing the gel cell on my belt or a shoulder attachment. I just don't know what amperage of gel cell to consider? Also would i need a means of limiting the current?

[Sacchetti]

There are several metal detectors on the market that take 8 AA penlight cells. I was considering making a mod where i could substitute a 12vdc gel cell to replace the AA batteries. I don't mind wearing the gel cell on my belt or a shoulder attachment. I just don't know what amperage of gel cell to consider? Also would i need a means of limiting the current?

Hi!
There is not necessity lo limit the current because the circuit get what it need to work.
It is important instead to have the same voltage, in this case 12V.
Remember when you have a PB Gel full charged do you have 14,4V.
The maximum voltage I seen with 8 new alkaline battery is instead 13,2V, but it depends also from brand and model.
You must be sure these 1,2V volt added do not damage your MD.
Do you have a schematic? Which is brand and model?

After this check if is possible to use it without problem then you can evaluate 1,3Ah or 2Ah model.
More Ah has the problem relative to the weight.
Less Ah has the problem relative to the time of working, it is reduced.
Therefore you must check which is the current absorbtion of your MD.
Normally, PI technology absorb more current if compared to IB technology.
These are generic indications, it is better also to measure the current absorbed from you MD.
Remember when it sound it is bigger.
The same if your MD has light in LCD, when you turn on it your MD get more current.

Personally, I do not like PB Gel battery because if you do not use them continuosly then you can meet problems relative to its sulfation.
You can found in internet some schematic circuits to solve this problem.
Normally they send high frequency electronic pulses.
A good one circuit was showed from elektor magazine in the past years but I do not remember in which month/year but I can search it if you are interested.

[6666]

Remember when you have a PB Gel full charged you may have 14,4V.

A couple of series diodes will fix that .

[Ham]

You are correct. It is not worth the chance of ruining a detector by using a gel cell battery. I don't know if eight rechargeable AA batteries would be practical? The expense of always buying eight alkaline batteries for a detector would also not be practical.

[6666]

I don't know if eight rechargeable AA batteries would be practical?

Yep

[Skippy]

You've still not told us what detector(s) you are talking about. Just about every one will likely work on 8 AA NiMH's. Failing that, there's the possibility of making up a 9-cell pack of different size NiMH's, eg. 2/3AA, just depends. Don't forget the weight of a battery pack can be a significant part of balancing a machine, eg. XP GoldMaxxPower/GMAXX, with the control box below the elbow cup. Removing 200 grams of battery won't help the balance.

[Ham]

I was considering the "Whites Coinmaster GT".

[Sacchetti]

A couple of series diodes will fix that .

These diodes have a forward voltage of about 0,6/0,7V, enough and correct for your application.
6666 is right, two diodes solve the problem.

If the MD absorb less of 1A you can use 1N400x family where x is a number for voltage but in this case also 1 alias 1N4001 (50V) it is enough.
Here the datasheet: http://www.vishay.com/docs/88503/1n4001.pdf
You can use also 1N4007 for example but 1N4001 it is enough for this scope.

If you want exagerate because you are using a strong Pulse Induction then you can use also 1N540x family.
It is 3A general purpose diode.
Here the datasheet: http://www.vishay.com/docs/88516/1n5400.pdf

If you want to use other diodes it is necessary to check always the forward voltage at the current where your MD operate.

If you think that strong PI do not exist then please visit www.pulsdetektor.de
I do not like this type of detectors, strong power without a good control, Miner 4 is an example.
Sorry if you do not share my opinion but take only as opinion, nothing else.
In other words I would like tell you there are some Pulse Induction with good software that absorb less if compared to those and the software has also discrimination.
But it is another discussion...

[Sacchetti]

There are several metal detectors on the market that take 8 AA penlight cells. I was considering making a mod where i could substitute a 12vdc gel cell to replace the AA batteries. I don't mind wearing the gel cell on my belt or a shoulder attachment. I just don't know what amperage of gel cell to consider? Also would i need a means of limiting the current?

Take attention:

You told me 8 AA cells but I have considered Alkaline battery.
8 alkaline battery produce 12V and 13,2V only when they are new (fully charged).

If you are speaking instead of 8 AA cells NiCD or NiMH the voltage is lower.
Your Metal Detektor use alkaline or rechargeable batteries NiMH technology?

[Sacchetti]

I was considering the "Whites Coinmaster GT".

This is the user manual of this detector:
http://www.kellycodetectors.com/pdf/...nmaster-gt.pdf

At page 5 there is written 8 AA Alkaline batteries.
But at page 6 there is explaind you can use also NiMH battery pack (I think they are 10 elements) but you must buy it as extra.
I think it is better to use alkaline batteries in this case.
You have maximum autonomy, you avoid the cost of extra pack and also to balance well the machine.

The only problem of alkaline battery is if you do not use them for a long time can produce acid damaging the battery holder and the near electronic.
Therefore remove them before to store your MD in your box.

Have a nice day!

[Ham]

Ok, thank you for the information.

[sinclairuser]

i have never been happy with performance when it comes to rechargeables, i have tried them all, untill recently i was using duracell alkalines( 24 cells for £9.99), a set of those(8cells) would do 3 hunts for me.
then at xmas my son gave me two packs of Eneloop rechargables, unlike nicads or nmh these run at 1.5v, and have been a revelation for me, they are expensive so you need to work out your battery costs to see if they are value for money for you.
if you buy a lot of cells they are worth considering, their performance is comparable to alkaline in my detectors and maybe slightly better.

[Ham]

The Eneloop rechargeables your son gave you, what are they in milliamps?

[Polymer]

I second what sinclairuser has stated, I was never happy with rechargeables until I got these.

I've been using 2nd generation eneloop AA NiMH'S 1900 mAh (3UTGA) for 2 1/2 years. Active usage up to now about 900 hrs.

I am VERY happy with them. The 4th generation is already out!

https://en.wikipedia.org/wiki/Eneloop

I charge them with 500mA.

If you choose to get re-chargeables do yourself a great favour and get an intelligent charger with delta t &or delta v (I'm sure someone can explain this better).
I've got the BC700.

Here's a pic of discharge curves alkaline vs. NiMH:



Taken from this pdf file which I find is a nice read:

1.5_vs_1.2_Volt_Batteries.pdf

[6666]

I also use NiZn @ 1.6 volts per cell, if you experiment long enough with all types of rechargeable cells you will discover the better ways to charge/maintain and use them to best advantage
and max performance.
Some of my cells are over 20 years old.
Have a look at a 5S 5AH lipo into a LDO 12 v reg, for light weight high capacity hip mount for serious detecting.