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I have heard the Energizer charger will overcharge. I'm guessing it's a dumb charger, with no charge state detection (I haven't used it myself). The Energizer NiMH cells aren't bad, but you could definitely use a better charger. And if you're getting a better charger, you might as well get better AA batteries too. I recommend you get these ones:

• New battery day! 24 AA Eneloop NiMH batteries in 16 and 8 cell packs. : r/AAMasterRace
  
  Get this package first to get the highest quality charger on the market:
  
  
• AmazonSmile: Panasonic K-KJ17MCA4BA Advanced Individual Cell Battery Charger Pack with 4 AA eneloop 2100 Cycle Rechargeable Batteries: PANASONIC: Electronics
  
  You need that charger to get the full life out of Eneloops. If you take care of them, Eneloops will last at least a decade, maybe longer.
  
  NiMH cells usually peak at about 1.45 volts when fully charged. Because of variations in voltage testing devices and their test probes, you could get slightly different numbers. The best chargers do not rely solely on peak voltage to detect the full charge state. They watch for a peak voltage, then a slight drop in voltage, and finally an increasing cell temperature. That's what the Eneloop charger does, and that's what most fancy aftermarket chargers do too. The difference is the Eneloop charger is much cheaper, so that's what I usually use.
  
  I have some fancy aftermarket chargers also, but I only use them if I run out of Eneloop chargers for a lot of cells, or if I need to use their analyzer features. One of them runs also on DC power, so I can use it with a solar panel (which I haven't tried yet). So, bottom line, you probably don't need anything other than the Eneloop charger.

If you want to build it yourself: you are on the right track.

If you want a (almost) ready solution, then this might be what you are looking for. It's a simple 11.1V - 3S case with (5V) buck converter and a BMS. You should test if your horn works down to 9V. If that's the case then this will be an excellent solution.

>Given the voltage and current requirements, should I expect to be able to build this in a convenient handheld size and weight form factor?

This will depend on your usage. You should test the horn and calculate it's power consumption.

>Does a 4S system sound reasonable here? If so what kind of battery protection and charge circuit am I looking for?

As stated above, you need to test your horn to test its voltage range.

>Can I reasonably charge a 4S system from USB? Is there a convenient boost converter board out there to do this, or built into a 4S battery management board?

For simplicity I would go for a dedicated 3S or 4S charger.

>Should I instead consider a single cell battery system with a DC-DC boost to drive the load? That would seem easier to charge and manage the battery, but 3.7V to 12-14V at 10A seems like it might be a hefty boost converter.

14V * 10A = 140W.

140W/3.0V = 46A

You'd need to draw 46A from your 1S pack of cells.

​

Not exactly your answer, but I highly recommend getting some quality low self-discharge NiMH rechargeable batteries. The Amazon Basic ones are good. Not only rechargeable, but they don't leak. Eneloop is the gold standard as well and they come with a decent charger.

You basically get a free charger with this pack:

Panasonic K-KJ17MCA4BA Advanced Individual Cell Battery Charger Pack with 4 AA eneloop 2100 Cycle Rechargeable Batteries https://www.amazon.com/dp/B00JHKSMJU/ref=cm_sw_r_cp_api_i_.aQ3Db9XMZ1SM

My original post is based on trying to replicate from pictures what someone else has done which, presumably, worked at some level. Metal detector technology, particularly in the battery department, is still in the stone age. Dealing with 8x AA cells is unreasonable and ends up with a ton of crap in the landfill, hence my wanting to swap out so something better.

So, if building something like this is complicated, unsafe, etc... what's the happy path to getting to where I want to be?

Something like https://www.amazon.com/TalentCell-Rechargeable-3000mAh-Lithium-External/dp/B01M7Z9Z1N

and setup an adapter from the current battery pack connector to convert to it?

To be clear, I'm not doing this because I have any desire to build a battery pack, I'm just trying to get somewhere more reasonable than a fistfull of AAs.

>https://www.amazon.com/dp/B01KYZ5R2G/ref=cm\_sw\_em\_r\_mt\_dp\_U\_5ySBCb2GB29DZ

Ah, thank a bunch. I just assumed that the charger would go to the battery leads. I think I get it now. Thanks for the help!

Would one hook up this board in the same manner? Charger to P+ / P-? I hope I didn't damage anything hooking it up how I did.

https://www.amazon.com/dp/B00JJIZYSC/ref=cm\_sw\_em\_r\_mt\_dp\_U\_8PYBCb1P7T78X

How many? If a bunch, your best bang for the buck is these:
http://www.hobbyking.com/hobbyking/store/__84348__Turnigy_Rechargeable_Battery_AAA_900mah_NiMH_4pc_US_Warehouse_.html
Fewer, you can probably save on shipping with these:
https://www.amazon.com/AmazonBasics-Rechargeable-Batteries-8-Pack-Pre-charged/dp/B00CWNMXQW/ref=sr_1_1?ie=UTF8&qid=1468606777&sr=8-1&keywords=nimh+aaa+amazonbasics

AAs available from both sites also.

yes absolutely.

you may lose some life if you don't use matched cells but this is COMMON

RC Car battery packs (before lithium) worked this way.

you had 7 and 8 cell sub c cell packs. any of those chargers will charge your pack just fine.

charge at 1amp should be plenty safe a decent charger will auto detect full and stop.

do be sure to charge them separately the first time then discharge and charge "as a pack" only so the cells don't mismatch.

this will work just fine

http://www.amazon.com/gp/product/B003MXMJX8/ref=pd_lpo_sbs_dp_ss_2?pf_rd_p=1944687602&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B002PECWXC&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1SVXNCYT30XW7S7HEZA8

and it is cheap too. you should be able to get away with a 2amp charge if your using 2amp hour or better cells.

>with 75min reserve capacity... yet it only has 66min reserve capacity.

That smells like an advertising gimmick, if you test it after charging the battery to 15.0V or 14.5V down to 10.5V then obviously you get longer service, I wouldn't worry too much about that.

>Whats the best deepcycle 12v battery i can get.

You could look at the size of the battery tray under the hood (and available height) if a larger battery will fit, that should improve reserve capacity.

Another thing to look at is the number of charge/discharge cycles, ...and 300+ discharge/recharge cycles, cheap to buy could mean expensive to replace often.

That is an interesting thought. For my lights that take 3 cells, I was thinking of getting several of these battery holders so that i could run eneloops in parallel and wire them to the lights. In a setup like this, I wonder if it would be worth putting an inline fuse between the packs and the light just for added safety? Or maybe there is a better strategy other than linking together battery holders.

Edit: If I'm wiring in battery holders, I suppose I could also just go with 3 D cells in a holder like this instead of bothering with AAs in parallel.

Edit2: One last crazy option came to mind. I could wire a 3 D cell holder to the lights, and then use 3 of these AA to D cell converters to use my existing Eneloops in parallel.

> https://www.amazon.com/Battery-Electric-AA-Converter-Convert/dp/B015T7TERI

That's perfect! Thanks so much!!

Batteries haven't swelled at all. They are reading 13V.

i did move them alittle to bring them up stairs and i wired them up.

I purchased this charger:

http://www.amazon.com/gp/product/B0000AXTUY/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1

Seems hopeful. Maybe i have charged them in the past year.

*Edit: I plan on hooking these up to a inverter this weekend so i can play videogames in the park for a few hours. Will i be able to?

Ah then you're in luck! In my experience, if you hit right voltage for little electric motors like this, they'll turn. If you can't supply enough power, they'll turn slowly but they'll work. Also, again these batteries might not last long (I would imagine minutes, compared to hours for AA batteries). I'm not sure exactly what you're looking for as a form factor but here are some options for small, high power, 3V batteries:

http://amzn.to/2r4m5yX

http://amzn.to/2sCz5xY

http://amzn.to/2s2JHZV (not necessarily high power but it might work)

I believe these are the droids you're looking for:

https://www.amazon.com/Battery-Electric-AA-Converter-Convert/dp/B015T7TERI

Keep in mind that you need to account for whether there's one battery, or two batteries in your device.. and if there's two batteries, whether they're in serial or parallel. There's a couple of these floating in amazon for each case iirc, so just buy the right one

For low voltage batteries, I recommend Barsukov's book (from Texas Instruments); "Battery Power Management for Portable Devices"

He means to use these batteries and these converters.

Normally I'd make the case for rechargeable AA cells but those get very expensive for occasional use electric candles.

Here your go: only 1kwh https://www.amazon.com/dp/B004LPIYJ4/ or here is one with 3.2kwh https://www.amazon.com/dp/B01M61VLBZ/

https://smile.amazon.de/gp/product/B01FX1LVQM/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1 It's this clock. The note with the battery is on the 5th picture (in English)

Hello again 1Davide, here's that link.
Gikfun 2S Lithium Input Ouput Protection Board PCB 7.4V 3A for Arduino (Pack of 2pcs) EK1785 https://www.amazon.com/dp/B01KYZ5R2G/ref=cm_sw_r_cp_apa_i_DM2vCbVX1JWA8

https://www.amazon.com/Enersys-Hawker-Cyclon-0810-0004-Battery/dp/B0002ILK86

https://www.amazon.com/gp/aw/d/B07TVLKMH6?ref=ppx_pt2_mob_b_prod_image&th=1&psc=1

Found it in english.

Mechanism looks like your basic one found in almost all battery powered clocks. Even if an Alkaline damaged it, a replacement costs about $2.

A picture shows it with a Duracell, which I don't think come any way but alkaline.

> But my question is, are there any major downsides to using rechargable batteries? When I was younger I used to be told that they don't hold their charge for anywhere near as long in storage and they deplete quicker when being used but are there any tests to back this up?

When we were younger (well, when I was anyway), rechargeable batteries sucked. They didn't hold their charge long at all in storage, and it didn't take long before they seemed to permanently lose their charge. There doesn't need to be any tests to back this up because it was obvious to anybody using them that they sucked. They are much better now, though. A standard rechargeable AA battery (Eneloop or one of the many generics) can be recharged 1,000 times and are slightly more expensive than alkalines but nowhere near 1,000 times as expensive. In my experience, they are a good value money-wise, and of course they cut down on waste which makes them better for the environment. A lot of people around here and /r/flashlight say that they actually perform better in high-performance applications as well, but I don't know enough about that to really be able to comment on it.

I'm in the process of switching my house to rechargeables and have more or less settled on these. They are made in Japan in the same factory as Eneloops but are cheaper. I'm not opposed to other brands at all, but these (in AA and AAA) are what I have been buying. I basically only use alkalines in applications where I'm not sure I'm going to get the batteries back and in very limited applications that call specifically for alkalines (my smoke detectors and thermostat both say "Alkaline batteries only" on them for some reason).