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Started 20Jun2025. Updated 25Jun2025. This note is in groups My e-fun notes (electronics fun) and Technology. Observe Standard disclaimer.

Intro

What may I use my box with «12V» LR20 alkaline batteries for? I do have direct «12V» output, but more, how could I use the quite nice USB A and USB C charger outputs [1]? This is not about power banks full of rechargeable batteries. This is supposed to be a log of an non-pretentious exercise with «recycle» (not recharge) after use, LR20 batteries.

→ Beware of the Conclusions (below). This was fun and I learned a lot. But useful only for some cases!

Diagram and build

The diagram is quite simple. The charger unit came with a 5×20 mm fuse holder with a slow 10A fuse. I replaced it with a slow 5A instead. The four twin battery boxes had connectors as on 9V (PP3) type batteries. The wire size of the connectors I purchased was too small, so I soldered in larger sized wires. Batteries like these would be able to start fires under the wrong conditions. Disclaimer: Make any such box on your own risk.

I also made a template plate at the bottom, to make sure that the rather heavy boxes with two batteries didn’t have free motion. Plus a couple of protecting walls. The display of [1] would drain the batteries if it weren’t for the switch. The unit weights 1.6 kg. I have a solid strap to hold it together. But beware, it’s a lab box, not for outdoor use.

Two AA-type internal 6V units

I already had these two. They are not sold any more. Probably because of little practical usage. Both take the four of smaller AA (called LR6 as alkalic) batteries. The one to the right also charges charges HR6 (NiMH) or KR6 (NiCd) type batteries. Nice to have, the white Exibel Portable Power (E-400-U) unit as Clas Ohlson (38-4518) or the black Biltema (24-0144) (MWBUAA44) unit.

As a reference, both will charge [Load-iPhoneXR] and [Load-radio], but the black unit will disconnect after 20 seconds when the load falls below 100 mA.

LEGO D-type internal 12V bank

I used the USB-tester [2]. The internal voltages here refer to the display’s values of the internal battery bank’s voltage.

The 5A slow fuse; I have neither tested nor found any data for its correct value (well, see below). I could test it with fresh batteries and it may blow, but then it may not blow on flatter batteries. But the Luxorparts charger has current limitation, which is nice. The purpose of the fuse is to protect a situation with a broken charger, plus a short on the banana plug outputs. Maybe a slow 2A fuse would have been more correct, since that’s closer to what a D battery would ever deliver. But then, that would fast conflict with this:

Using the [Load-iPhoneXR] I saw that I was able to charge it over:

1. USB-A gave me 5.07V @0.99A, with battery bank voltage of 11.1V
2. USB-C gave me 4.95V @1.2A, with battery bank voltage of 10.8V

I particularly wanted to test it with the more demanding [Load-MacBook]:

1. USB-C did not to deliver. I think it tried to deliver 5.09V, without success
2. USB-A gave me 5.08V @1.4A, with battery bank voltage 10.8V. This was used to power the MacBook
3. However, when I configured the MacBook Battery state Low Power Mode from «only when battery» to «always» the USB-A also charged the battery. After some 10 minutes and 217mAh delivered the percentage on the MacBook went from 32% to 33%. At that time:
   USB-A gave me 5.09V @1.4A, with battery bank voltage 10.4V. This was used to charge the MacBook
4. After this setting of the battery state I saw this when 100% on the MacBook’s battery:
   USB-C gave me around 11V when less than some @0.3A, with battery bank voltage 12V. This was used to power the MacBook. Depending on how much current the MacBook drew the USB-C was completely switched off much of the time.
5. USB-C and MagSafe 3 in the MacBook; same result as point 1 above.

Using the [Load-radio] I saw that I was able to charge it over:

1. USB-A gave me 5.04V @low current (it had almost 100%), with battery bank voltage of 11.9V
2. USB-C gave me 4.92V @zero current (it now had 100%) with battery bank voltage of 12V

Discussion

My purpose wasn’t to do much in-depth here. I have not checked how it would behave if I had 16 batteries * 1.5V = 24V and then tested the USB-C.

A battery has a capacity that depends on how it’s used. It’s not like a bucketful of 10 litre water. What one can do with the battery depends on how fast and how often it’s being emptied. Therefore Wikipedia (D battery) cites a capacity of LR20 of 12,000–18,000 mAh and 18–27 Wh. per battery. Of course the make also makes a difference, and then probably the price.

Multiplied with 8 we’d have 96,000 -144,000 mAh and 144-216 Wh. Bear in mind the you are only allowed to have hand baggage with a power bank of max. 100 Wh on an airplane, so this box is rather powerful, or more correctly «energyful».

Allowing the voltage to go all the way down to 0.8V per cell (6.4V in our bank), according to [3] if constant current:

1. 0.25 A in 52 h = 13 Ah
2. 0.5 A in 20 h = 10 Ah
3. 1 A in 7 h = 7 Ah
4. 2 A in 1.8 h = 3.6 Ah

There are similar tables for constant power and constant resistive load. Even constant temperature @250mA. Example

1. 250 mW in 61 h = 15 Wh
2. 100 mW in 7 h = 7 Wh
3. 10 Ω for 130 h
4. 21° for 51 h

Aside: Alkaline D-type LR20 batteries are alkaline-manganese dioxide (Zn/MnO₂) batteries and dry electrolyte. See Wikipedia (Alkaline battery). I read there that the nickel-iron type batteries were precursors, with wet electrolyte, and that they were invented by Thomas A. Edison in 1901. I remember when my father got us two of these large and heavy beasts, used from railway cars, in the 1960’ies, for our cabin. We had two battery arrays in wooden frames, and we carried them with the rather lightweight Hillman Minx (series V?) car home again, to be charged once a year. For some years they did supply us with power for some incandescent lamps, enough for sporadic use when we were there.

Loads

→ The problem with my loads is that they basically charge internal batteries. But most gadgets have internal, rechargeable batteries these days! But some units might take external power and disconnect internal non-rechargeable batteries in the connector. These units would be almost ideal for my box!

[Load-iPhoneXR] iPhone XR (A2105) (2018).

[Load-MacBook] Apple MacBook Air 13″ M3 (2024), which accepts power through a MagSafe 3 connector in the machine (with a USB-C connector at the charger end) or directly to either of the two USB-C ports in the machine.

[Load-radio] Soundmaster IR6500SW radio. See My soundmaster radio IR6500SW modified

Conclusions

Even if I am not a battery expert I try these conclusive points:

1. This box is not of much use for charging my laptop or even mobile since these units slurp more than the batteries would appreciate. It kind of «works», but would be expensive and polluting. Use a rechargeable power bank instead
2. This indicates that there is a basic mismatch between the batteries’ max current and USB-C and even USB-A max currents. Battery + USB charging don’t tango
3. For smaller loads my box may be useful. Which probably means a unit that doesn’t have any internal battery of its own or disconnects it when externally powered
4. Since these type of batteries are sold almost everywhere, even an empty box of mine might become handy on occasions
5. Decreasing the mismatch with more current (for charging) could be to connect batteries in parallel. One with one, two with two or 8 with 8. But parallelising batteries is not advised, as slightly unequal batteries would cause idle current between any pair. The pairs would self drain faster then necessary. They too don’t tango
6. Also a USB-A output like [4] draws about 40 mA @12V. This indicates that the internal electronics in these gadgets use about this amount, and that the internal voltmeter of [1] has little mA cost, so to say.

A new box?

From the last point above I conclude that the USB output unit isn’t necessary if the unit to be powered takes, like 6V, since the internal batteries would be the same. Not «USB 5V» so to say. This would be the ideal solution. So a new box should have a separate single pole switch for the USB output unit and a double throw switch for 6V and 12V directly from the batteries. Plus a common fuse in the gnd path. Something like in this diagram – which also opens for an unfused 6V up the chain.

References

Wiki-refs: Alkaline battery . D battery (LR20). USB Power Delivery (PD). Quick Charge (Qualcom QC)

[1] Luxorparts USB charger for panel mounting (USB-lader for panelmontering) 30 W. 12V/24V input. See www.kjell.com/no/..40754. Claimed to have USB-C (PD Power Delivery) or USB-A (QC Quick Charge 3.0). Like [4] it draws some 30-40 mA @12V in idle mode.

[2] Uni-T UT658D USB-tester, see www.kjell.com/no/ ..40729.

[3] MN1300 Duracell. See https://www.electricalworld.com/Images/ModelDescriptions/Duracell-Plus-Power-D-LR20-MN1300-Specification-electricalworld.pdf

[4] Biltema USB-A 12/24V in, double USB-A out. See https://www.biltema.no/ ..43-202. Has a built-in LED. Like [1] it draws some 30-40 mA @12V in idle mode