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mz-rh10 battery sidecar (a bit long)

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I've piled through the archives for info, but still have some doubts/questions about this arrangement. Sony gives very little info about the AA sidecar. The manual basically says to screw it on and stick an alkaline AA cell in. If you just want a summary, go to the bottom of this note and read there. If you want some gory details, read on.

I found the service manual on the site (thanks, btw) and had a look at the schematic. I had assumed that the internal nimh and the external sidecar were isolated electrically somehow. I was completely wrong. The sidecar contacts on the side of my rh10 are directly connected to the internal battery. If you have a volt meter, you can measure the internal battery voltage right on those contacts.

So, this is where the difficulties come in for me. I know a bit about batteries and electrical design using rechargables. I've never seen a commercial product that direclty parallels two different types of batteries with two different voltages. So why did sony do it? Actually, it seems to work too, so what's the problem?

After some looking, it appears to be a simple and elegant solution.

First, a fully charged NiMH battery will have a terminal voltage around 1.5 volts. This is very close to the terminal voltage of a fresh alkaline battery. So, if you have just completed charging your internal NiMH battery, then add the sidecar, very little "cross charging" will take place. If your internal battery is flat and you add a fresh AA to the sidecar, the internal battery WILL charge from the alkaline. However, the charging will not be anywhere near complete. This is where it gets interesting...

The charge and discharge characteristics of NiMH batteries are very different from an alkaline battery. During discharge, the NiMH battery quickly drops from its static full charge voltage of about 1.5 to about 1.3 volts. Then it stabilizes during discharge and hangs near 1.2 volt until the battery is just about exhausted, then the voltage drops quickly. The alkaline battery is quite the opposite. Its voltage drops steadily during discharge. The alkaline at 1.2 volts is only about half exhausted.

So, looking at the possible scenarios:

Internal battery fully charged and a fresh alkaline in the sidecar:

The batteries will not affect each other much. NiMH batteries lose their charge quickly, so if you leave the batteries like this, the internal battery will slowly kill your alkaline (over several weeks), but shouldn't damage the internal battery at all. During discharge, the alkaline battery will supply the bulk of the power initially, but as its voltage drops to 1.3 volts the internal battery will pick up more of a share, eventually supplying most of the power to the player. Once the internal battery is exhausted, the alkaline will probably hold up the player for a little longer, as it can supply power at lower voltages. Eventually they will both be exhausted.

Internal battery drained and a fresh alkaline in the sidecar:

Initially, the alkaline will begin to charge the internal battery. However, the charging voltage of the internal NiMH will quickly rise to 1.4 volts after maybe a 10% charge. The alkalines voltage will begin to drop fairly quickly as it supplies power to the internal battery to charge it. Eventually they will equalize with between a 10%-20% charge on the internal battery and a decrease in the terminal voltage of the alkaline. This will still work, but it isn't very efficient, as the charge process is not 100% efficient. Either way, the player will be powered mostly by the alkaline, and slightly by the charge in the internal battery. If I were to guess, I'd say that you would probably lose 10% of the alkaline power that you would have gotten if the dead gumstick battery were removed.

So, we know (I guess we already knew) that this scheme works. Now why did Sony leave out the rechargable AA as an option for the sidecar? I think it was for a few reasons. First, an alkaline battery will never back-charge from the internal cell. Secondly, NiMH cells have a very low internal internal resistance and can supply a tremendous amount of current if short circuited. If shorted, they can burn (fire) or burst. If you had a NiMH in the sidecar in your pocket and something shorted the contacts, you could get burned badly.

Also, if your internal battery is dead, and you put a fully charged AA NiMH in the sidecar, the sidecar battery could try to charge the internal battery quickly. This could, possibly, damage the internal battery by charging it too quickly without any charge control. This is the one thing that would make me wary of using a NiMH in the sidecar (with a dead gumstick). Honestly, I'd need to take a longer look at this to see if there is really any issue.

NiMH discharging in parallel, even at different capacities, is fine. They will share the load until depleted. Charging in parallel is a no-no, but since the player will never be able to charge the sidecar, this is not an issue.

Sooooo, to wrap it all up:

1) Alkalines work fine in the sidecar.

2) Adding an alkaline to the sidecar when the internal NiMH is dead will work, but is likely a little less efficient because of back-charging.

3) A rechargable NiMH AA will likely work fine in the sidecar, and likely give you improved performance over an alkaline (assuming that you are using a high capacity NiMH over 2000mAH). Just be careful with the NiMH AA's. Use a battery case. If you short them out, you'll be sorry. I would avoid putting a fresh NiMH in the sidecar if the internal battery is dead, unless I was sure that it would not damage the internal battery with a brief but large initial charge.

Sorry this was so long. I was trying to be thorough. BTW, these are just my opinions. Do what you think is right.


Mike M.

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An excellent informative post. I rarely use my battery packs actually, due to fear of reducing the life of the gumstick. I may start though, only when the internal is fully charged. It'll mean less charging of it anyway, so maybe even increased life that way?

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Well, I did a quick experiment this evening, trying to see what kind of cross-charging would take place with a depleted NiMH cell in parallel with both a fresh alkaline and a fully charged NiMH. I'm sure my current meter affected the results, but in general the cross-charge current was an initial 250mA. Likely not enough to damage the internal battery. However, whatever power is transferred to the internal battery will be at a loss. Battery charging is much less than 100% efficient. Also, I believe that the NiMH will transfer more of its charge to the internal battery than a alkaline would. If I have some time this weekend I'll try another experiment.

For me, if I ever need to replace the sidecar battery and the internal battery is dead, I'll remove the internal battery before installing a new sidecar cell.

Hope this helps.

Mike M.

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Thank you for this great post :ok: - I assume this is not just applicable to the RH10 but any unit that uses a gumstick / sidecar combo (NH900 / EH70 etc..) - is that right?

* Moving to Tips & Tricks

It should be the same, but to be sure I'd have to look at the schematics for those units.

Are they on the site somewhere?


Mike M.

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