Important : Most Revos are very reliable and only a few users experience problems with battery life. This Information is provided for the unlucky few and those who want the very best life from their Revo batteries.

Psion Revo/Revo Plus - Charging Information

I've heard quite a bit of misinformation about the Psion Revo with regard to the batteries and decided to post this information to clear up a few issues. Important: I think this information is sound but I would welcome any additional information or comments - use at your own risk.

How It's Supposed to Work

The Revo is equipped with two AAA NiMH rechargeable batteries and an internal charging circuit. This circuit is fed by a 6V DC adapter, usually by the docking port.

In theory, when you put the Revo on charge it should fast-charge (300-450mA) displaying red and green LEDs until 85% is reached. Then it should switch down to a trickle charge (~30mA), displaying just the green LED, until 100% is achieved.

The internal threshold for 'Low Battery' is set to <10% and the 'Recharge' message appears at <3%. Sound is available until the 'Recharge' message appears. Both thresholds are dependent on calculations based on the charge calibration point (more on this below).

Contrary to popular and nervous opinion, the Revo charging circuit is a precision piece of kit and uses purpose-built charging ICs designed by one of the world's leading battery manufacturers. However, to work properly, it is vital that the user follows the first charge instructions to the letter.

The battery level meter works in the following way :-

1. The first time the Revo is charged from new, or after a hard reset, the charge controller senses a chemical change in the cells. It does this by detecting a sudden thermal change in the pack which occurs at 89% of actual (84% of the indicated) capacity. This is a feature of NiMH cells. We'll call this the charge calibration point. It is absolutely crucial that you follow the Psion guidelines for the first charge so that the controller can detect this accurately.
2. During normal charging or running, an accurate current reading is obtained using a 'gas gauge'. By measuring the current in and out of the cells, the controller knows where the battery level is in relation to the original charge.
3. There is a 30% reserve capacity on the Revo, ie. when you have 0% left on the indicator there is still 30% capacity left in the cells to retain the memory.
4. When you charge the Revo normally there are actually several things that can trigger the drop-down to trickle charge:-

• Detection of the charge calibration point.
• Overall Timeout
• A thermistor on the cells reaching a preset temperature.
• If the Revo is put on charge at a capacity higher than the charge calibration point, it will switch down after approximately 5 minutes.

Possible Problems

• When your Revo is brand new or has a hard reset, the charging circuit doesn't know what the true battery level is and resets it to zero. It must be charged to 100% after a hard reset so that the level accuracy is restored - this can take up to 6 hours.
• The charging circuit is notorious for not switching down to the trickle level when it should. This happens especially if you recharge the Revo when the battery level is already in excess of 84% though the figure will vary if it hasn't been calibrated (charged) correctly.
• If the Revo batteries get hot, a thermistor kicks in and shuts off the charge circuit -this may be the reason why some people have noticed the level draining when they have left the Revo on the docking station for a long time although there is also a timed switch-off even on trickle charge.
• There was a problem with a small number of Revos (8MB, not the new 16MB Plus version) that had faulty battery packs. The cells in these were less than a tight fit and could shake loose causing the terminals to make intermittent contact. This caused machines to reset themselves without warning, losing all data. Psion's fix for this was to replace the slightly undersize cells and fix the new ones securely with a dab of glue.
• On older Revos another problem has been discovered. The battery connection cable can wear leading to a high resistance that fools the machine into switching off during high-load conditions (such as playing sound), even though the battery level is over 50%. D&G Upgrades have a fix for this potential problem and also do other enhancements including memory upgrades.

About NiMH Cells in General.

Nickel metal hydride rechargeable cells offer the following benefits over NiCd :-

• Higher Capacity
• More useful life (eg. for memory retention when too low to power the machine normally)

The trade-off for these benefits is :-

• Lower number of charge/discharge cycles (about 300-500 as opposed to 1000 for NiCds)
• No memory effect
• Lower maximum current (not disadvantageous for a palmtop computer)
• More easily damaged by overcharging.
• Higher self discharge (NiMH ~2%/day, NiCd ~1%/day)

The Revo is never really off so a little power is used each day just retaining the clock and memory etc. Psion's estimate of memory retention while the Revo is unused is 6 weeks maximum when fully charged or 2.4%/day on average. ChaDis assumes a little heavier usage of 3%/day in its estimate of memory retention time during periods of zero use. If you have a Revo+ or Mako then you should assume 6% - this is due to twice as much memory that has to be powered even when the machine is 'off'. A 28MB memory enhanced Revo+ loses about 10%/day for the same reason.

It is important to note that the symptoms of overcharging NiMH cells are reduced capacity which looks remarkably similar to the 'memory-effect' in NiCds. This is the reason why you will see people reporting that NiMH cells still have a memory effect - don't believe them - it's overcharging !

When your NiMH cells get old they will lose capacity anyway, but this should only be of the order 10-15% reduction.

Revo Battery Recovery

If you find that your Revo batteries are not retaining their charge that well, it's possible that they have been damaged by overcharging but it is far more likely that the charge calibration point is incorrect. However, all is not lost !:-

1. Check your battery performance using ChaDis. It will try to ignore any periods while the machine is off by using previous figures obtained for that particular 1% level drop step.
2. If you have lost significant capacity, backup your Revo and then let it drain until it switches off (turn off the auto-off features). Then perform a hard reset and charge the Revo in accordance with Psion's first-time charge instructions, ie. 6 hours.
   Now this bit is very important ! - turn the machine OFF during this 6hr charge (or at least pay no attention to the gauge). Do not be tempted to ignore this if the gauge apparently reads full.
   This may put everything back as it should be and you need take no further action except to restore your files.
3. If this doesn't work, drain the cells again and recharge to 20% and then drain again. Recharge to 40% and allow to drain. Repeat this a few times in 20% intervals until 100% is achieved. Then repeat step 2 except for the backup (which you've already done) to recalibrate the gauge. The repeat + 20% process should recover most of the capacity of NiMH cells that have been degraded due to overcharging. The reset and complete recharge then recalibrate the battery level gauge.
4. Allow the Revo to drain until it switches off and make a note of how long this takes. If this exceeds 8 hours (for screen-on but no activity) then you can be certain that your Revo is working correctly again.
5. If you've carried out these steps and still can't get a sensible time then your batteries are beyond recovery You will need to have your Revo repaired by Psion or POS Ltd. If your machine is under a year old then Psion will repair it under warranty.
6. If your Revo is out of warranty and you are fairly competent, you can replace them by removing the Revo badge on the top of the machine and moving just two pins (see here ).

Revo Battery Charging Tips

1. Always run ChaDis and recharge with your Revo switched on - if your Revo charging circuit carries on fast charging when 100% has been reached, at least you'll know about it and can take it off charge or consider a hard reset and full recharge to recalibrate the gauge.
2. Try to get into a pattern of charging at set intervals as is practical for you and avoid charging when already over 84%. Any level between 10% and 80% is fine. If your Revo switches over to trickle charge then leave it on for an hour or two, otherwise take it off within 5-10 minutes of 100% (audible ChaDis warning) - it should switch down itself within about 5 minutes anyway.
3. Remember that it is completely normal for the machine to lose 3%/day (6% for Revo+/Mako) when unused and plan for periods of inactivity where possible.
4. The 10-12 hours usage claimed by Psion appears only achievable if your Revo is sitting switched on but hardly used continuously. 8 hours is a far more realistic target for real use.
5. Backup regularly !

General Comment

It's a pity that Psion didn't put the NiMH cells under a simple battery cover with a single retaining screw. If they had, people wouldn't get as nervous about them. I also think that the first-charge recommendation should be given far more prominence - ideally with a big label on the top of the docking station !

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Sunday, 25 May 2003