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I want to replace a battery pack of 6 AA 1.5V alkaline batteries arranged in series, with rechargeable NiMH batteries in an old transistor megaphone in such a way as to optimise the capacity. The existing battery pack is a typical 2×3 arrangement and it is connected with a PP3/PP6 snap connector. I want to use rechargeable batteries to reduce electronic waste.

I'm aware that usable capacity is subject to the device's cut-off voltage, the discharge curve, and the efficiency of the batteries given the temperature and load characteristics. Given that this is an old device and probably not designed with a low cut-off having rechargeable 1.2V batteries in mind, I'm concerned that 6 cells isn't enough and want to include a 7th.

Although I'm aware that rated capacity is a generally misleading figure, I don't understand why my naïve calculations seem to be several times out from what I'd expect judging roughly by the mass/volume of the batteries. Are PP3s really that bad?

In the battery compartment there is space for 4 of the following 5 arrangements (see note 2 below) for which I'm demonstrating these naïve, seemingly false calculations. (In the case of arrangement #4, there is also space for a small adaptor I made using 3 snap connectors required to connect the batteries in parallel.) This is roughly related to how ‘big’ the batteries are and relates loosely to their volume, so these arrangements are likely to be applicable to other devices as well.

(#)  Arrangement                    Combined       Charge    Total energy
                                    rated voltage  capacity  capacity[1]
(1)  6 AA 1.5V alkaline in series   9V             2100mAh   15.12Wh = 54.432kJ
(2)  6 AA 1.2V NiMH in series       7.2V           2400mAh   17.28Wh = 62.208kJ
(3)  7 AAA 1.2V NiMH in series      8.4V           950mAh    7.98Wh = 28.728kJ
(4)  2 PP3 8.4V NiMH in parallel    8.4V           200mAh    3.36Wh = 12.096kJ
(5)  14 AAA 1.2V NiMH; 2 series[2]  8.4V           950mAh    15.96Wh = 57.456kJ
  • Note 1: The ‘Total energy capacity’ is a naïve estimation based on the charge capacity and 1.2V per cell mean voltage.
  • Note 2: 14 AAA batteries in a (1+√3)×5 arrangement (i.e. rows of 5, 4, 5) can occupy approximately the same cuboidal space as 2×3 AA batteries. I've included this for the general case; however, in this instance the battery compartment has rounded corners so the arrangement wouldn't quite fit.

That is not what I'd expect at all – not even close; PP3s look to be about 2 and a bit times the volume of a AA, so I'd expect the total energy capacity of arrangement #4 to be about 70% that of #2, but this rough estimation suggests 19%! I'm guessing that the rated charge capacity is rated in respect to a ‘typical’ load, so the efficiency of each rating is probably vastly different, and therefore can't be used to reliably determine the actual energy storage capacity which would be underestimated by a different amount in each case. In the case of arrangement #4, the combined internal resistance is halved (i.e. ‘internal conductance’ added), so it would be more efficient and would deliver more energy per battery given the same load, as compared to just a single PP3. In arrangements 1–3 the IRs are added. Arrangement #5 has half the combined IR as #3.

For what it's worth, the megaphone is a pulsing load rated 5W maximum power draw, and it's operated outdoors in the UK so the batteries probably generally operate with a temperature between 5°C and 25°C depending on the time of year. I'm trying to find datasheets for the NiMH batteries I use (Maplin L32BJ, L29BJ, and L31BJ for AA, AAA, and PP3 resp.) to actually see for myself the true data of what's going on with these figures, but I haven't yet obtained any. However, specifics aside, these figures seem to be so far out that a better general understanding of how to compare rechargeable replacements for battery packs of 6 AA alkaline batteries would be generally useful because I've seen such battery packs on other types of devices as well.

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    \$\begingroup\$ This is my first question on this site. I fear it might be borderline ‘too localised’, but I've tried hard to keep the overall scope more general than my particular instance of comparing these sort of ‘9V’ arrangements. I hope it's okay. \$\endgroup\$ – James Haigh Feb 18 '14 at 2:14
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    \$\begingroup\$ Did you test out just using 6 straight NiMH's to replace the alkalines? They may work better than you expect, even if the device wasn't designed to accept them -- Alkalines will dip below 1.2V when about half the capacity is used up, NiMH's have a much flatter curve and will maintain 1.2V longer. \$\endgroup\$ – Johnny Feb 18 '14 at 3:18
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    \$\begingroup\$ AAA are from the pit of hell. PP3 are worse unless you must sell your sould for the compactness they offer. What mean or loaded voltage you get depends on load. Megaphone is perhaps heavier than some. If it takes say 5 Watts then draw approaches an amp as battery gets down. NimH are generally happier under higher loads towards the bottom of their capacity - ie they will sag less from Voc at low charge levels. Alkaline starts at just over 1.6V/cell, rapidly falls under 1.5V and then falls in an S type curve (lying backwards, tall and then, squint eyes to see it) ... \$\endgroup\$ – Russell McMahon Feb 18 '14 at 4:52
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    \$\begingroup\$ @JamesHaigh I don't follow your 109% x 6/7 argument if standard cells are used. If the cells had to ft in a fixed space then adding another cell would reduce capacity per cell. But, if AA cells were used then adding #7 increases capacity x 7/6 to start. | A 7th cell only makes sense if Vmin_operating is more than about say 6.6V. In this case a primary (eg Alkaline) cell would discharge to 1.1V and have used mosts of its capacity, whereas 6 x NimH at 1.1V still have more useful capacity left. 6.6V/7 = 0.94V/cell which is indeed lower than is wise with NimH. 7 x 1V = 7V gives a useful Vmin. \$\endgroup\$ – Russell McMahon Feb 20 '14 at 10:22
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    \$\begingroup\$ @JamesHaigh "Motorboating" is low frequency oscillation of an audio amplifier which occurs when battery impedance rises towards the end of its discharge cycle. Variations in current drain in one part of the circuit cause the supply voltage to vary due to the increasing impedance and the voltage changes affect other parts of the circuit - effectively being coupled by the battery impedance. If this occurs in the "right" location oscillation can occur. Adding a large capacitor to the supply reduces the impedance (or can be seen as removing voltage variations)(same thing). \$\endgroup\$ – Russell McMahon Feb 20 '14 at 10:27
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It sounds like you may have a handle on your question already, but I would like to add a couple of points about NIMH vs Alkaline.

You don't say how many watts or how many amps or milliamps your megaphone will be demanding, but the NiMH have a much lower internal resistance and so they can provide a much higher current without dropping their voltage as much as an Alkaline will. At higher loads, a Nimh will provide more power than a Alkaline; at very low loads, an Alkaline will provide more power (a very very loose generalization, but for example, an Alkaline will last forever in a remote control while the NiMH will not last as long - of course there is also the self discharge of many of the NiMH).

Also, most NiMH have a NOMINAL volatage of 1.2, but fully charged they start out at closer to 1.4 (I have measured some NiMH fully charged at 1.5) volts, and as mentioned already, hold to the 1.2 volts for most of their discharge.

The Alkalines often start closer to 1.6 volts, but quickly lose voltage as they discharge so that their average voltage through their life is about 1.2 Volts! Of course this depends on what the cut off voltage of your Megaphone is.

Don't forget that most NiMH have a very high self discharge rate and can lose 10-20% of their capacity in the first day, and 1% of their capacity per day just sitting there and are essentially completely discharged in 3 months or less! There are newer NiMH that hold 85% of their capacity over a year.

Check out http://batteryuniversity.com/learn/article/Nickel_based_batteries for more info.

As far as your 4 options, option 2 sounds like the best one. Options 3 and 4 have been discussed by others as to why they offer such poor performance.

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    \$\begingroup\$ Looking at the discharge plots that Johnny linked and elsewhere, and seeing that they are roughly rotationally symmetric but slightly more rounded at the low-charge end, the mean voltage really does seem to be practically the same at about 1.2–1.24V for both NiMH and alkaline, which means that my energy capacity calculations are actually pretty good approximations. So I've now filled in values for #1 based on Spehro's answer and 1.2V. I really don't understand why alkaline batteries are rated 1.5V! Especially now you say they often start higher than 1.5V. Mean voltage is less misleading. \$\endgroup\$ – James Haigh Feb 20 '14 at 10:52
  • \$\begingroup\$ I was aware of self-discharge. I've asked Maplin to provide figures for self-discharge on some of their batteries for comparison. Since they've recently started selling batteries branded as ‘hybrid’, which are just pre-charged low-self-discharge NiMH batteries with a higher price-tag, I've been asking for clarification by providing proper data. I've also asked for datasheets for some of their batteries, but as of yet, nothing. \$\endgroup\$ – James Haigh Feb 20 '14 at 11:29
  • \$\begingroup\$ It's a good idea too get the data sheet on these 'hybrid' batteries. It seems like brands that I have seen that call their low self discharge NiMH 'hybrid' tend to have a slightly higher self discharge than the other low elf discharge batteries. Also, in general, low self discharge NiMH will have a lower mAh capacity than the regular self discharge of the same brand - since the purpose of this question is to optimize the capacity for your megaphone.... \$\endgroup\$ – Filek Feb 23 '14 at 7:45
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Part of the reason PP3s don't have a good volumetric efficiency is that it uses cylindrical cells inside of the PP3 case. You can't easily make alkaline or NiMH batteries in any other form factor than a cylinder due to the need for compression of the electrolyte. That said, some companies have figured out ways to pack alkaline batteries in a stack making efficient use of the space in a PP3, but they still seem to have the same capacity so I am thinking they did it for ease of assembly rather than giving more bang.

Also there is a greater wastage of the zinc used in the case for smaller battery sizes. The zinc case makes up one of the electrodes and must be of a certain thickness to physically hold the cell together and not be eaten through by the electrolyte (which is why cheap batteries with thin shells eventually leak). This is much thicker than is needed for it to act as the negative electrode. In smaller batteries there is a higher percentage of space used by the zinc shell relative to the amount of electrolyte.

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  • \$\begingroup\$ Thank you. This has clarified what I suspected about cell-size, though it seems to be more of a factor than I initially thought. I new that some PP3s had cylindrical cells, but I was guessing that most modern ones use stacked-cells. Maybe they do, but like you say, they mostly don't seem to be trying to use this arrangement to pack the capacity. I did see a 300mAh PP3 NiMH battery, though it's the same price as a 3-pack of the 200mAh PP3s. There must be quite a bit of space lost having 7 cylindrical cells because it doesn't seem like a great number to try to pack into the PP3 form-factor. … \$\endgroup\$ – James Haigh Feb 20 '14 at 5:31
  • \$\begingroup\$ …This got me thinking about how I'd actually arrange those 7 AAA batteries in #3, and I realised that actually 14 AAA batteries can be arranged to occupy approximately the same cuboidal space as would 2×3 AAs! This gives an energy capacity of 15.96Wh (57.456kJ) which is remarkably close, and can be connected to form the 8.4V I was initially looking for. However, due to rounded corners of the battery compartment, this arrangement wouldn't quite fit for the megaphone, but I'll add it to my question anyway in case of potential relevance to other devices. … \$\endgroup\$ – James Haigh Feb 20 '14 at 5:45
  • \$\begingroup\$ …Although it's an interesting arrangement, it would require making a custom battery pack and is still likely to decrease the running time for most devices. I think there would be some specific cases where there would be a marginal improvement, but having looked at those plots, I think it would be very marginal and not worth it, so I still prefer #2. \$\endgroup\$ – James Haigh Feb 20 '14 at 5:46
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Just doing a rough calculation of comparable technology Panasonic batteries operated at 30mA constant draw, I get 15h for the 9V and 70 hours for the 1.5V AA cell. Given that the 9V has 6 cells, that's 28% more energy than in a single AA cell.

They weigh about double, so the mass energy density is about 2/3 as good for the 9V battery. Not surprising since it has many cell walls and such like inside.

The real killer is the $ energy density. In 100 quantity, the AA cell is $0.35, and the 9V battery is $1.90, so the cost of running on 9V batteries is going to be 5-10x higher.

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  • \$\begingroup\$ Okay, so #4 probably has about 40–50% of the energy capacity as that of #2. That figure makes a bit more sense compared to 19%, but I assumed that a self-contained multi-cell battery would be able to pack the cells more efficiently than having a number of individual single-cell batteries in a battery-pack. The price is less of a concern for rechargeable batteries since they last a lot longer for what they're worth, but it's still a shame it's such a difference. \$\endgroup\$ – James Haigh Feb 18 '14 at 3:17
  • \$\begingroup\$ I watched a video which though slightly inaccurate in places, I found quite informative (and also rather amusing), and learnt that some devices have such a high cut-off that they <Aussie>“just piss away”</Aussie> over half the capacity, so there's still a small possibility that #4 may still be the most suitable, but I seriously doubt it now. Thanks for the info. \$\endgroup\$ – James Haigh Feb 18 '14 at 3:19
  • \$\begingroup\$ I found out that charge capacity ratings are measured at low currents such as these, and are therefore a good indication of the upper limit. I don't think my calculations are quite as naïve as I thought; PP3s are just really really really bad when they are NiMH, probably due to space inefficiency of fitting 7 cells, yet they're also pretty bad when they're alkaline as well. It's quite astonishing that for NiMH, 3 AAs can store more energy than 5 PP3s! And even with your alkaline figures of 2100mAh and 450mAh, the PP3 like you say only has about 29% more energy capacity for all its extra bulk. \$\endgroup\$ – James Haigh Feb 20 '14 at 11:00
  • \$\begingroup\$ There has been a lot of competition to build really high capacity AA cells as well. The Ah are often prominently displayed on the package for retail sales. \$\endgroup\$ – Spehro Pefhany Feb 20 '14 at 12:26

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