Vamo problem

[raid2506]

Dec 3, 2012 17:11:25 GMT dave said:

I've noticed a strange problem with one of my Vamo's. If it is left unused with a battery in it, the battery slowly discharges. It is only by 0.1 volts in about 5 hours, so would be easy to miss. I only discovered this because it was unused for several days and the battery had gone down below 2 volts! I monitored it after that with different batteries which is how I came up with the figure above. I haven't checked the other one I have yet, so I don't know if this is a general problem or if I just have a faulty unit. Anyone else noticed this?

Avoiding all the maths Dave, it sounds like a dodgy/dirty switch in the unit to me.

[dave]

Well this has led to some very enlightening info about batteries! It is the V2 and I did check a battery in it with the unit switched off - the battery still discharged, albeit at half the rate it did when the unit was switched on. Clearly a fully charged good quality battery shouldn't be discharging at even a quarter of a volt a day, so there is something wrong with the unit. I did wonder about giving it an Isopropyl bath as it may very well be gunge in the innards, but since it is a brand new unit I think I will just return it. I was surprised to discover from the PO website that I should be able to post it back to China for about £4, so that looks like the best plan.

[azzie]

Dec 5, 2012 15:50:05 GMT dave said:

Well this has led to some very enlightening info about batteries! It is the V2 and I did check a battery in it with the unit switched off - the battery still discharged, albeit at half the rate it did when the unit was switched on. Clearly a fully charged good quality battery shouldn't be discharging at even a quarter of a volt a day, so there is something wrong with the unit. I did wonder about giving it an Isopropyl bath as it may very well be gunge in the innards, but since it is a brand new unit I think I will just return it. I was surprised to discover from the PO website that I should be able to post it back to China for about £4, so that looks like the best plan.

Good to hear you can get it sorted relatively cheaply Dave, £4 to China is nothing, I dont understand how it can be so cheap when it`ll have to go through Royalmail first..on a plane and then through mainland. ???

Which vendor did you get the unit from?

[dave]

I got this from HC. I was just quoting the international small packet charge for less than 150gms given on the PO website. I was really surprised it was that low as well - of course it more than doubles if you want recorded. The PO have a really useful tool you can use to find out the postage cost of anything to anywhere:
www.postoffice.co.uk/price-finder

[alvoram]

Dec 5, 2012 8:19:40 GMT maccafan said:

Dec 4, 2012 23:14:09 GMT alvoram said:

But are they not measured and rated to take into consideration that they shouldn't be used below 2.75? If not then that is a bit misleading IMO.

Yes..I did not explain that very well..

But once they go below 3.2-3.3 volts they hardly give out any energy and plummet to 2.75 volts..This energy discharge graph at 0.2 amps (200 mA) shows how they start dipping at the beginning and the end.



What I am trying to get through is you cannot take a battery capacity or voltage and work out the time it is going to last for a given load the way you are doing it by simple multiplication and division. The energy it gives out is not proportional through it's discharge curve at safe voltages. At 4.2 volts the plummet is quite quick and then it stabilizes at around 4.0 volts and then again starts to plummet at around 3.3 volts and then undervolt if it is not protected.

The capacity is calculated from 4.2 to 2.75 but the battery becomes almost unusable ie., flat below 3.3 volts..Any load and the voltage will just plummet without much energy being given out..

Thanks macca for taking the time to try and explain, I had seen charts like this before, and that's kind of what I was getting at, that the batteries had almost reached their tested usable capacities before they dropped off. Take the tenergy 2600 as an example (the easiest for me to read) that has a capacity of what looks to be 7800 mWh to 7900mWh up to the point it drops to 3.2 Volts.

So based on your calculations from your previous post, at 0.15 mA consumption, the highest consumption in terms of wattage would be when the battery was fully charged, at 4.2 x 0.15 = 0.63 mW.... with that consumption dropping down as the battery drops in voltage, your sum was 3 x 0.15=0.45 mW.... so ignoring the curve, or put simpler, ignoring the fact that the consumption drops as the battery loses charge, and calculating it based on the highest consumption, should yield a result, in hours, that is less than the true length of time, in hours, it would take the battery to discharge at o.15mA.

Based on this, when I calculate that 7800mW / 0.63mW (the highest consumption remember) and I get 12,380 hours, the actual result if I factored in the reduction in consumption as the voltage drops, and followed the curve, should actually be more hours than that.

See that is why I am confused, not because of the curve, but because you said that the reduction in consumption in terms of mW as the battery voltage drops, (0.63mW at 4.2V as opposed to 0.45 at 3V) would mean the amount of hours we'd get from a 7'800mWh battery at 0.15mA would be less than the figure I got from calculating at 0.63mW, when simple maths suggests it would actually be more.

Put even simpler I'm struggling to see how a gradual decline from 0.63mW to 0.45mW demand on a battery would lead to the battery lasting less time than if there was a constant 0.63mW demand on it. ??? ??? ???

[maccafan]

I can see what you are saying..I need to look into the self discharge rates of Li-Ion in addition to quiescent current..It may be higher than what I thought it was.

The reason I said it won't last a year and 2 months (10,000 hours) is because we use large amounts of Li-Ion cells in our trains and the recommendation from SAFT is to top them up to 40% charge every 3 months if they are in storage..

Will speak to someone from work who has forgotten more about batteries than what many people know about them.

[vapee]

Yea I thought the higher mah ones would last for ages but they did some testing on one of the forums and like not all of them seem to actually be true to their mah rating, also I used to run them down to 3.4v and one time I think I ran it down to 3.3v and suddenly saw it dropping to 3.0v, when I came to charge it, it made some odd sound like a kettle boiling so I gave up on that battery and I won't trust these batteries to drop too low anymore
Might have not been the best quality battery tho, was a sanyo I think
Also I read about these rechargables leaking some kinda substance after a while, and it sounded like this could happen within that 10,000 hour period
I guess its best to have a fair bit of batteries, but not too many lol