Li-Ion Battery Life on OSTC Sport

[R4cOOn]

Hi,

I've got an OSTC sport (hardware D292-0E06).
I use a Li-Ion battery (battery type T2).

It's a bit of a tight fit and I've got to force the battery a bit to get it to slide in.
After about 2 weeks with no diving, the battery dies.
The charge indicator goes from 100% to 75% and then the battery dies.
I was expecting the battery indicator to go all the way to 0% just before dying.

Is this the expected battery drain from the unit?
The original disposable lithium battery lasted about 5 months and 30 dives.
My last dive was in September and in December the cell was dead.
I was expecting at least the same kind of endurance with a rechargeable battery.

I've got 2 different brands of Li-Ion batteries rated at 850mAh and they both behave the same.

I'm a bit worried that I'll be diving when the batteries dies.

Cheers,
--
Arnaud

[Leonvank]

The thing with Li-ion cells is that they remain at pretty much the same voltage throughout their working life and when they reach the end of their capacity, their voltage drops rapidly.



So what happens is that it will happily swing around 70% charge and suddenly drop to 20% in a relatively short time. When that happens, you'll need to recharge the battery/computer to avoid having the computer die on you while diving.

The reason for the difference between rechargeable and non-rechargeable batteries is that the non-rechargeable variants can contain a lot more energy when compared to their rechargeable brethren.

[abcdef]

I see in hwos source code that for devices without battery gauge IC such as OSTC sport, the voltage is used to calculate the percentage.

src/hwos.inc gives percentage levels in mV:

#DEFINE lithium_36v_75 .3000 ; [mV]
#DEFINE lithium_36v_50 .2900 ; [mV]
#DEFINE lithium_36v_25 .2600 ; [mV]
#DEFINE lithium_36v_10 .2500 ; [mV]

Compare that to your voltage curve above, or to the Saft LS14500 data sheet, and you'll see that the numbers are a bit off...
A 3.6V Saft LS14500 discharging at 8.6mA stays above 3.4V for some 250hrs. At 3.0V it's already at its end of lifetime, not at 75% remaining. Discharging with 55mA, 3.2V should be the 10% level. I don't understand where the numbers in hwos.inc come from, they seem too small for me.

-------------

For 1.5V batteries, it seems the formula in src/adc_lightsensor.asm is (x-1.1V)/0.4V*100%. So 1.3V means 50%. This is OK for Alkaline, but in the Energizer Lithium datasheet this is too optimistic and 1.3V should be more like 20%.

[abcdef]

In the Energizer Lithium datasheet referenced in src/hwos.inc, we can see this curve and correct my statement above:

[attachment=0]energ_EBC402R.png[/attachment]

With the formula in src/adc_lightsensor.asm, the voltage levels translate about like this for battery type T0:
1.45V: 88% is lifetime 35% lithium
1.40V: 75% is lifetime 20% lithium
1.35V: 63% is lifetime 10% lithium
1.30V: 50% is almost empty lithium

HWOS seems to use the same battery type T0 for both 1.5V alkaline and 1.5V lithium, and does not distinguish between them. Other vendor's computer has two different battery type settings for these two with different levels of "low battery" warning, and it's important to pick the right type.
The battery_warn_level in hwos.inc is fixed at 25% (1.2V), which is OK for alkali (10hrs left) but very low for lithium (1hr left). The battery_warn_level for lithium better be higher in hwos too.

HWOS displays battery voltage levels in percentage which users may confuse for remaining lifetime. This could be documented in the manual, pointing out the different behavior of lithium vs alkaline.

[abcdef]

back to the 3.6v batteries
looked at the histories, it seems the voltage levels were higher before and got reduced around revision 77 of ostc3.inc in Feb'2014.
Before Feb'2014 the 75% level was 3.2V which fits well with the Saft LS14500 datasheet (after 20 out of 30 hours with 55mA).
The values in hwos.inc match the Saft LS14250 at 55mA, but this battery can't be used in OSTC sport. These values fit the wrong battery with too high current load.
So I guess some time 6 years ago somebody reduced the 75% level from 3.2V to 3.0V in ostc3.inc, later copied to hwos.inc. The consequence is that since that time, the battery indicator drops too late to only 75% for an empty Saft LS14500 and then the battery dies, as the OP describes. A low battery warning at 2.5V may be too late.

[PTUK]

All of the above battery info is quite technical, albeit, I get it now but only by having a 12v car battery explained to me sometime ago...........12v car battery needs to be about 13/13.5v minimum to work, best is 14.8V but if it's showing 12v, it is too "flat" to work properly!!!!!
The 12v is what it gives out when fully charged to e.g. 14.8V