Need help making a 12V battery

**petn7** · 04-20-2008, 08:54 PM

Re: Need help making a 12V battery

Hello,

I need to get more run time out of my boats. I run 2 x 6v SLA;s in series. 6v to RC and motor. 12v to pump. I'm needing a change....

I want to make a 12v battery out of 10 1.2v Nimh or Nicds. I just don't know what connectors to use and what type of charger. It needs to run a 12v pump (sheerline setup). I can easily arrange them in series. I am looking at buying 10 1.2 v 2500Mah or 2100Mah Nimh batteries. I will be making a watertight plastic casing for them. My boats are really small and there are simply no commercially available 12v batts that small.

1. Do they let off heat or gas? Will this be a problem in a sealed compartment?

2. What type of charger would I need. The only ones I seem to find are the "plug in the individual AA type" ones.

The rest of my boat runs off a 6v SLA.

any pics ,diagrams and general comments owuld be greatly appreciated.

regards,

David H

A good R/C charger should work. Finding a charger that handles 10 cells won't come as cheaply as a "budget" 2-8 cell charger, though.

I could be wrong, but sealing the batteries probably isn't a good thing. (I assume the batteries never leave the plastic container?) I'm guessing rechargeable batteries have holes in them for a reason.

Also, a set of Deans connectors should work. Amp draw is more of an issue with plugs than voltage is.

**anonymous** · 04-21-2008, 04:07 AM

I have recently been using

I have recently been using a 12v Lipo in my !:32 Holland class which I am building. No problems in my early pond trials.

It was for all the same sort of reasons that you describe; need for 12v: small size; availablity etc.

I know all the horror stories with Lipos but we have had discussion in these forums all of which give a qualified "Go for it".

You may wish to put Lipos on your list of options.

Davy

**anonymous** · 04-21-2008, 04:09 AM

I forgot to memtion that

I forgot to memtion that to get 12v you only ned a 3 cell Lipo not the 10 cells for NiMh.

Davy

**Guest** · 04-21-2008, 04:28 AM

Splitting hairs a 3-cell li-po

Splitting hairs a 3-cell li-po pack will give 11.1 volts, slightly more more for a fresh charged pack, but that will soon drop.

Bearing in mind that the PWM action of a modern ESC isn't 100% efficient (probably close to 85-90%) you may want to consider a 4-cell li-po pack-14.8 volts for a 12 volt motor.

Just be sure the electronics can handle the extra voltage though, many can't.

Andy

**anonymous** · 04-21-2008, 06:25 AM

Thanks yes, you are right

Thanks yes, you are right Andy.
3 cells seems to work OK but go for 4 cells if you are buying a Lipo.

I use a switch mode BEC (to reduce power losses) from
www.dimensionengineering.com

with a home brewed low voltage alarm.

Dimension do lovely little BECs and also Lipo voltage alarms.

Davy

**bradv** · 04-21-2008, 06:58 AM

Hi Dave http://www.subcommittee.com/forum/icon_smile.gif

Sounds like

Hi Dave

Sounds like you want to do something similar to my little Kilo, where I've got a small 7.2v NiMH pack (mostly) sealed in the front. In answer to your questions:

1. NiMH packs let off heat & gas (hydrogen) when you charge them and also if you discharge them really rapidly. Particularly for charging, it's thus not a good idea to seal a NiMH pack in the same compartment as your motor as a spark when starting .... well, I'm sure you can imagine it I use a bulkhead between my battery compartment and the rest of the WTC with the power wires siliconed through it. This prevents the hydrogen from getting back to the motor. When charging, I open a vent (made from a soft-drink bottle top!) in my battery compartment to let the hydrogen out. As long as you charge at a reasonable rate, the heat build-up is no problem. Same for running & discharging, but you have the advantage of water-cooling the compartment!

2. I use a standard NiMH charger which takes banana plugs in its outputs, and then wire these to a small suitable plug connected to my battery pack. But as noted by others, some chargers max out at 8 cells See your local hobby shop or John Darke regarding this. I use a tiny 2-pin header connector on the sub, and matching plug, to charge through.

Important
If you want to build a NiMH pack, make sure you get solder tag cells! Not only does this make it much easier to build a pack, but prevents the heat from soldering onto a standard "nipple" end from damaging the cell. Try these]http://www.jaycar.com.au/productView.asp?ID=SB1728&CATID=18&keywords=&SPECI AL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pag eNumber=&priceMin=&priceMax=&SUBCATID=583[/url]
You can then wrap them in heat-shrink tube like this to make a nice pack!

http://www.jaycar.com.au/productView.asp?ID=WH5580&CATID=&keywords=heatshrink&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

Another point - I've found that occasionally one of the rechargable cells I buy won't actually charge up. These are easy to tell on purchase if you take along a multimeter and put it across the terminals - any cells with 0v are stuffed. The few cells I've had like this just will not charge successfully. As long as a cell is holding a reasonable voltage at purchase, not even the full 1.25v, it's probably OK.

Note that if you go the LiPO route, a balancer will most likely be required as well as a special LiPO charger or you'll get sheets of flame that will do very bad things to your sub! Jim has had 2 go up violently at his shop - one as a test of the charging pot, the other not!!

Good luck with your endeavours!

**david h** · 04-22-2008, 07:13 PM

Weeny 12v battery

Thankyou everybody for the prompt replys,

After reading all the responses about the different battery options I decided to take a further look at the humble SLA. I searched on the net and found an SLA that is the same size as my 6v SLA's that I run in series that produces 12v It is the PS 1208

http://www.master-instruments.com.au/pr ... S1208.html

Catch is that it's only 800Mah. So being the Electronics dummy that I am I presume that it will run the pump, just not for long. How long is the question. The pump will only run for 10 seconds max , at a time and then only a couple of times, I tend to run submerged mostly.

The pump that i use has the following specs,

Voltage Current drain

No load 12VDC 320ma
Max efficincy 12VDC 1.68A
Load Constant 12VDC 1.8A-2A

Does all this make sense? I will be stoked if i can get this pump to run for 10 seconds every so often for up to an hour.

please enlighten me,

regards,

David H

**bradv** · 04-22-2008, 10:39 PM

Hi Dave,

For a fully-charged 800mAh

Hi Dave,

For a fully-charged 800mAh battery running the pump at full 2A current draw, the maximum continuous runtime you'd get in minutes is:

(0.8 / 2 ) * 60 = 24 mins

Now, as you use the battery its voltage will drop and will eventually become too low for the 12v pump. SLA's have a mostly linear discharge until near the end of their cycle when it drops off much more quickly, so (finger in the air) lets assume your pump will only operate effectively for 2/3 of the discharge time. You'd then be able to pump continuously for:

2/3 * 24 = 16 minutes

As you'll only be using the pump for maybe 30 seconds maximum at a time (overestimating), you should get a minimum of:

16 / 0.5 = 32 full pump cycles

As you can see, I've taken maximum values to try to cater for worst-case. This will obviously be 3 times more (eg. 96) if you only pump for 10s as you mentioned - this will depend on the size of the tank. And if you use a valve to let the water out, this will be up to 96 tank fills.

Hopefully this'll be adequate for your needs!

Need help making a 12V battery

Need help making a 12V battery

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