/ Help needed making a homemade AA battery pack

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Ash Routen 19 Oct 2019

Hi folks,

I'm looking to make a homemade battery charger (using AAs) to charge a sat phone, mobile etc. It's for an Arctic trip.

I have bought this 8 x AA battery holder with a DC cable: https://www.amazon.co.uk/dp/B00LB76HFS?ref_=pe_3187911_248764861_302_E_DDE_dt_1

I've also got a female DC to USB converter: https://www.amazon.co.uk/dp/B076Q7WBG3?ref_=pe_3187911_248764861_302_E_DDE_dt_1

But on reflection I'm assuming I will have too much power output (8 AA x1.5v=12v) which would damage a phone battery or similar?

How can I reduce the output from the bank of AA's and make it safe to charge basic electronic devices?

Many thanks,

Ash

Mike Stretford 19 Oct 2019
In reply to Ash Routen:

> How can I reduce the output from the bank of AA's and make it safe to charge basic electronic devices?

You could make 2 battery packs at 6V each. The device would probably handle 6V (usual disclaimers apply) but the problem is that when the battery voltage drops below 1.25V (5V for 4), the device will stop charging. Alkalines start out at 1.5V but drop down as they drain

https://www.powerstream.com/AA-tests.htm

The better option would be to buy a 5V USB regulator, which will drop the voltage from anything above approx 7V to 5.2V (you need a bit more than 5V for charging but a USB advertised product should do that). There will be loads of these online of varying price/quality. I would be careful to get one with decent reviews. A decent one will handle a range of voltages,  it should work with 8 1.5Vs or 8 1.2Vs (NiMH).

This link should get you started

https://www.google.com/search?safe=off&rlz=1C1TEUA_enGB633GB633&sxsrf=ACYBGNQcKFfFEVvUYVX8TrDjEZy1f4z3fQ%3A1571503229170&ei=fTyrXe-ACpGN1fAPtcOz4Ak&q=DC-DC+Step+Down+Converter+5V+usb&oq=DC-DC+Step+Down+Converter+5V+usb&gs_l=psy-ab.3..0i8i30l2.7233.7233..7482...0.2..0.64.64.1......0....1..gws-wiz.......0i71.OMt7LXjbV40&ved=0ahUKEwivs_Pd4ajlAhWRRhUIHbXhDJwQ4dUDCAs&uact=5

I guess you want to use an AA pack as these are easy to get hold of wherever you are?

Post edited at 17:42
Martin W 19 Oct 2019
In reply to Ash Routen:

One of these: https://www.amazon.co.uk/JZK-Converter-Voltage-Connectors-charging/dp/B06XSCCLCD (other, similar products are available) will convert 12v from a car battery to 5v for USB charging. I've no idea how efficient they might be, though.  They're fine for powering/charging USB appliances from a car's 12v system (I use one to power my dashcam) but it's possible that they would be inefficient and wasteful of the limited capacity of a smaller power source such as a battery of alkaline AA cells.

Ash Routen 19 Oct 2019
In reply to Mike Stretford:

Many thanks Mike that's exactly the sort of thing I was thinking of.

I'll be skiing along the coast of Baffin Island. Solar panels are a hit and miss as we all know, and most power banks aren't much good in -40 unless you constantly keep them on your body.

 

malk 19 Oct 2019
In reply to Ash Routen:

what makes you think your diy attempt will fare any better in cold conditions? the capacity (mAh) isn't going to be large enough for charging phones with them in series. the way to go with diy powerbanks seems to be to use 3.6v 186500 batteries (eg old laptop batteries) in parallel and then step up to 5v. eg https://www.youtube.com/watch?v=Dk_HBqKl5Qg

Post edited at 18:51
wilkie14c 19 Oct 2019
In reply to malk:

yes this, buy a power bank caddy and fit your own batteries, 18650 or even better, 26650. Only buy quality, ie samsung 25r etc. skimp and buy cheap and you’ll regret it as soooo many batteries out there’s fake. it’s big business! 

Ash Routen 19 Oct 2019
In reply to malk:

Because I'll take about 80+ Lithium AA cells which I know work quite well when re-warmed after being cold, better than a power bank. Plus if you're one or two power banks drain early on in a trip you're stuffed. I'll be skiing for 40-45 days. A lot of arctic travellers make their whole system AA dependent.

I've seen folks charge sat phones, mobiles and iPods using a homemade 12v batt pack. Human Edge Tech used to make a good one. 

malk 20 Oct 2019
In reply to Ash Routen:

interesting, thanks. you could experiment with a car usb socket to see how many phone recharges you get. the trouble with series connection is the capacity stays the same- not much for a single aa

if you used multiple battery holders you could connect your 80 batteries in 8x10 series/parallel configuration to increase capacity to ~20000mAh

alternatively you could try connecting groups of 3 in series (~4.5v)/ parallel and using a step up usb board to 5v (more efficient?) so eg 12 batteries (3x4) would give ~8000mAh

https://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations

Post edited at 11:38
malk 20 Oct 2019
In reply to Ash Routen:

this gizmo steps down and up (input 4-13V, output 0.5-30V): https://www.amazon.co.uk/TeOhk-Supply-Boost-Buck-Adjustable-Converter/dp/B07SSX1418/

Frank R. 20 Oct 2019
In reply to Ash Routen:

LiFeS2 (the usual lithium AA primaries) cells perform well in the cold, but their useful capacity drops, especially with higher loads: (https://data.energizer.com/pdfs/lithiuml91l92_appman.pdf). Conversely, the cells do get warmer as they discharge. Better not loose that heat and insulate your battery holder. But the cells do have a built-in thermal shutdown switch (usually around 85C), while you are probably unlikely to trip that in normal use, it is a consideration if you have the box really well insulated and the cells under high load. Test it beforehand

If you need to do any soldering in your setup, select the proper solder - normal (mostly tin) solders are only good for normal temperatures, tin gets "brittle" in very cold conditions (as the Scott expedition allegedly discovered to their peril).

Anyways, you would need a DC to DC (5V USB?) converter. There are many types of those - pulse or continuous, boost type, etc. Unfortunately I am not able to provide a tip for the best one for your application - you might try asking around on some flashlight geeks forums like candlepowerforum or similar or your local electronics geek  

Consider diversifying your power - if most of your devices need charging (not running on the AA cells directly), take a solar panel with some lightening of your battery haul. It's always better to not rely on just one power source and electronics box that could fail. 

You could also consider other battery chemistries and industrial cells if your devices permit it - the LiFeS2 gives 1.5V, but there are other lithium primary cells (CR123A?) that work well in the cold and have higher voltage (which is usually better for the converter). Military uses LiSO2. But that's another can of worms - these are restricted for transport...

Jon Greengrass 21 Oct 2019
In reply to Ash Routen:

What about alternative power sources like a small wind turbine on your sled or a wind up radio like that invented by Trevor Bayliss, the company he started is still making them

https://www.freeplayenergy.com/products

EdS 21 Oct 2019
In reply to Ash Routen:

Why not buy one of the AA powered power banks?

Hooo 21 Oct 2019
In reply to Ash Routen:

Your plan sounds like good one to me. Take lots of separate batteries and swap them out, much better than a single large capacity power bank because if something goes wrong and it drains the battery you will have spares.

A few in series with a step-down converter is a good plan too. Battery capacity is usually better at lower current, and you can drain the last drop from the batteries. If you put four in series for 6v and charged your phone, it would stop charging with the cells at about 1.2V - with capacity in them that you can't use. If you make an 8 cell 12V pack they will drop out at about 0.9V per cell. It might be worth going up to 12 cells in your packs to get a little bit more.

A decent step down board will get to about 95% efficient, so definitely a gain. They weigh a few grams and cost a few quid, so take a spare or two. I bought some nice ones recently that do intelligent charging for newer phones. They set the output voltage at a value the phone requests, rather than fixing at 5V. I think they were a tenner for 3.


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