**Originally posted 4/15/2009 on AirsoftMechanics**
So I'm looking for a low voltage alarm for my Li-Polymer batteries since they can get permanently damaged if they go below 3 volts per cell. This crazy guy called WolfDragon tells me he can build one, and here we are literally 2 weeks later and he's already sent me a beta board to test.
I introduce to you the WolfDragon LiPO PCB Lite Alarm. The version I received has an audible Piezo buzzer alarm which retails for $17.50. He will also offer a version with a visible LED alarm for $15.
Don't get confused when you see similar products on the market, even for as cheap as $5-10. Those are made for the RC car crowd and our energy usage requirements are a lot different. With those cheaper models you can easily set off the alarm every trigger pull because the voltage can momentarily drop below 3 volts per cell when under drain (especially in high power guns). This computerized version WolfDragon made doesn't do this and monitors the voltage looking for the moment the battery truly drops below is safety point.
The product comes to you in two small packages:
The left package has the board, buzzer alarm, shrink wrap to cover everything, and a sticker with the version information. The right package has the plug that goes on the balance port of your LiPo battery complete with wires, along with an extra female port if your battery is missing it, and some shrink wrap.
Close-up of the alarm board front and back. It's approximately 23 x 16 x 6mm in size
Size comparison to some 6mm BB's and a penny.
Installing the buzzer alarm is simple. The wires are color coded, and the board is marked with A+ and A- (alarm positive and alarm negative wire). Since I wanted this as compact as possible I removed the wire and soldered the alarm directly onto the board.
Installing the balance plug wires is also simple. Black wire goes on the "G" (Ground/Negative), and for a three cell pack (11.1v) I installed the other wires in sequence on B1, B2, and B3. A 2-cell/7.4 pack would go on G B1 & B2, and a 4-cell/14.8v pack on G B1 B2 B3 B4.
This is the board installed on an Intellect 11.1v 20C 1400mah battery (if you're wondering the tan colored part is a 30 amp resettable fuse courtesy of Gandolf).
When the board is first plugged in the buzzer goes off twice with an intermittent LED flash, then flashes three times and you know the board is working properly.
I requested this board to fit in incredibly small spaces since I only use Lipo's when I'm really cramped for space. This shows how an 11.1v 1400mah LiPo, with 30 amp automatically resettable fuse, and a low voltage alarm all fit in the buffer tube of a standard M4 rifle (don't mind the butchered stock, that's an old project).
My other gun which is tight on space is a VFC AIMS AK47; which holds the battery in the same space as any other folding stock AK does (along the top of the gun), and this alarm still fits perfectly.
My first test was to use a fully charged battery. My gun is shooting 400 FPS with .2's at 30 RPS and I dumped an entire 300 round hi-cap in one trigger pull and the alarm did not go off. I proceeded to shoot another 300 round hi-cap with 100+ shots in semi-auto in very quick succession, then mixed it up with semi-auto and bursts and the alarm did not go off.
I then discharged the 11.1v battery to 9.1 volts (when I snapped the picture it dropped to 9.04 volts. On the first shot the alarm went off just as it should. I charged it up a little more to 9.2 volts and for a little bit (before it dropped below 9.1 volts) the alarm wouldn't go off. It seems to be very precise in it's voltage sensing as advertised.
I will be doing real world torture tests when I attend Battle For Tolland and Op Pine Plains in May, and will be trying this out in my 540 FPS with .25's semi-auto DM rifle as well.
This board seems perfect and extremely reasonable in price considering if you drop the voltage of your pack the first time you will damage it and can loose mah capacity forever (these 1400mah batteries cost $50 each for example and I would hate to damage one of my $80+ LiPo packs because of something stupid like a few extra shots which drop the battery below 3 volts per cell).
My only advice is to use a very fine soldering tip; the connections on the board are very small and if you use a larger tip, or have the soldering iron too hot you may run into problems.
If you're wondering why to purchase this alarm where there are other non-Airsoft-specific alarms for $5-10, then the answer is simple; those commonly give annoying false voltage readings because our batteries can drop to below 3 volts per cell for a split second since our guns pull a lot more amps on the first shot (read this post for more first hand experience with that problem: http://forums.airsoftmechanics.com/i...p?topic=1801.0
). This alarm doesn't have that problem since it's programmed to read the true voltage of the battery pack when it's not under strain.
I switched in a 600 FPS Guarder SP170 spring into my DM rifle and did some test firing. Shot about 1,000 rounds and the alarm didn't chirp one bit. Tried to do 50 shots in under a minute semi-auto, and ran some 100 round bursts to torture test the device. No dice. It works as advertised.
The real test will be next weekend at Battle For Tolland/Tribal Wars in Massachusetts when I'll surely use up two to three batteries in short order.
I went through 2 batteries and 6,000+ rounds at Battle For Tolland in MA this weekend. The alarm went off perfectly when it was time to change both batteries. When I went to charge the batteries the next day they were right above 9 volts like they should be (and above the danger zone). This is a keeper and I'll be getting a couple extras for my other guns when they're released.
The boards are now out of beta phase and ready to order (if the mods deem this advertising by posting a link of where to get this product, please remove this latest update).