PPP - Power for the Palm

Power for the Palm

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Before you read the following, I want you to remind, that all my tests were done with the newer PalmPilots. As I mentioned already on my serial port page, it looks like 3COM/USR changed the chip supplier for the the serial driver as well as the power supply converter. I refer to PalmPilots with the Linear Technology step-up converter LT1304. I've never checked the old Palms, but they seem to work with MAXIM IC's. Maybe they got kicked because of their bad quality data sheets :-) . I've read alot about the power supply of the Palm, the use of rechargeables and problems about data loss when changing the batteries. So here are my 2 cent ...
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.Rechargeables

.Changing the batteries

.The 'Don't-push-a-button-while-changing-batteries' fairy tale

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Rechargeables
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The power supply concept of the Palm was a very pleasant surprise for me. The use of a step-up voltage converter that operates down to below 1.5V not only guarantees the full 'exploit' of normal batteries, but also makes it possible to use rechargeables with a usual lower cell voltage. And the converter makes full use of the charge, what's important to prevent the well known memory effect of older NiCads (as long as you don't use a special and more expensive charger, that discharges the cells before charging them). Anyway, I use only rechargeables and I'm very happy with it. The type I use are the much better NiMHs and they are produced by Panasonic. Their nominal capacity is 500mAh, but I even get ~550mAh with a good charger. This is about 60% of good alkalines, or around 20 hrs of mixed operation time. I have two sets, swapping them about every week. Especially with the PalmPilot, it's a *real* good feeling to use the backlight as long as I want, without that strange feeling, that I have to pay for every minute (don't you know it???). The NiMHs here in Germany are about 4 times the price of a Duracell alkalines. But with a good (controlled) charger, you get easily 500-1000 charges, so it's really *cheap*! I've heard that these cells are hard to find in the States, but I don't believe that they are not sold over there. Panasonic is a major player in the rechargeable market, they should be available nearly everywhere. For Germans: CONRAD ELECTRONIC lists some suitable types. Check for example order # 250108-62 or # 250028-62. IMHO, NiCads are really not very good for the Palm. First, they have only ~250mAh which is indeed a bit less and second - more important - NiCads have a much 'harder' voltage characteristic than NiMHs. Due to the higher impedance of NiMHs (more like 'normal' batteries, what makes them bad for high current applications), the end towards 2V (with 2 cells) is slower and more predictable than with NiCads. With the NiMHs I use, I get a low voltage warning at least 1 or 2 hours before they really shut down (but then without backlight!). With NiCads that time might be very short, I've never tried it.
I've never tried renewables, I think they are not as good as NiMH. Hard to maintain, mostly more expensive (in respect to their lifetime) and most important, you don't need them in the Palm. Renewables were invented as a rechargeable alternative for devices that need the 1.5V /cell. Most annoying the fact, that you need to take care not to 'over-discharge' (or is it called 'under-discharge'?) them.
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Changing the batteries
. Important: I don't want to encourage anyone to mess around with his data! Obviously there are some Palms out there, that loose data quite quickly without batteries (below 30 seconds). Frankly, I don't understand that, I still think that this behaviour is faulty. But at least one user I know of, lost his data after he read that page. So, if you're not absolutely sure, that you can restore everything, don't change your good experiences on changing your batteries. Still, all results below are double-checked and true. .
Some people reported data loss after a batterie change, even when it was done quickly. That made me curious, and I opened my Palm another time to check, how good the data retention really is. As everyone knows now, the Palm uses 'only' a capacitor to supply the RAM and parts of the processor (real time clock and some registers). I personally like that concept, I'm tired of the 1001 different backup batteries I have to take care of in all the 'toys' I have. And I hope I can prove with the following, that there is no need of a battery. The cap does a goes job (normally for a lifetime), and I claim that Palms that loose memory within seconds are simply broken (or at least the capacitor). The capacitor is *always* charged to a voltage level of 3.3V, no matter how high the battery voltage is (as long as it is above 1.5V). I read wild speculations about one-after-another battery change for a better charge with one fresh battery already in. Fact is, the cap is charged with the output voltage of the converter and that is working as long as it gets 1.5V. The moment the battery is out, the cap delivers the current for data retention and is discharged. The safe low limit for most RAMs is 2V.
I recorded the voltage at the capacitor for my two Palms. One is equipped with 2MB original USR pseudo static RAM (4 chips) (note, after confusing a reader: I expanded that board myself from an original USR professional 1MB using the *same* RAMs, USR uses. I also know, that USR is not producing RAMs!), that is known for a higher standby current. The other one is equipped with a 3MB TRG board with 'true' static RAM (6 chips) that needs much lesser standby current:
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On 0 minutes I took the batteries out, before that time you can see the cap beeing charged to constant 3.3V. After that you can see the reduction of the voltage, depending on the current the system needs. The 2MB pseudo static RAM reaches its safe point after about 11 minutes and I put back the battery. You can see the cap beeing charged immediately again. Using the same time scale, the TRG true static RAM used much lesser current, leaving the cap charged to ~3V after 14 minutes! Just for fun, I recorded also the TRG board until it reached the 2V, and it gave me 80 minutes of backup time!
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Needless to say, that both Palms showed *no* data loss. I would say that with a 'healthy' unit, you get a multiple of one minute time to change batteries. Interesting the TRG board: With 3MB the most RAM and you still would have time to *buy* the batteries *after* you took them out :). Interesting also, that the real time clock in the DragonBall (after all a 'dynamic' device) influences the consumption much lesser than the type of RAM.
A final word on data loss: My speculation is, that a spike on the power supply of the Palm is more likely the reason for a data loss when changing batteries than a low voltage situation (unless there is a real defect) So IMO, changing the batteries one-by-one is not a good idea, as you have two times the critical situation of breaking supply and making 'first contact' again (no, your Palm is not assimilated ...). Maybe the worst is, when you've a 'shaky' hand when inserting the last battery (no joke!).
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The 'Don't-push-a-button-while-changing-batteries' fairy tale
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I'm sorry to express it that way, but hey, that's my site and a benefit of the work it takes to maintain it is, to be able to state things how I see them :-) It's pure nonsense when people claim they pushed a button while the batteries were out and ... boom ... capacitor discharged, data gone! Whithout the batteries neither the step-up regulator nor the rest of the logic won't start. So there can't be any activity that empties the cap. BTW, I tried it and as expected, nothing happened! As I mentioned on different occasions, there may be (better: for sure there are) defective Palms out there, that behave strange. They are perfect candidates for a service, but miserable ones for horror stories!
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