nimh not lithium.

Building something with a 3.6v nimh phone battery inside it and need to be able to have a wall charger. Something im working on cant say.
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Tim Team Reaper.

phone bomb :O
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A journey of a million miles begins with a single step followed by a hell of a lot of other steps so get walking

Nothin illegal just confidential.
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Tim Team Reaper.

you could charge them with a regulated power supply of the right voltage but youd still need to keep your eye on the temperature to know when to cut the charge off. im pretty sure neither are too convinient

nah has to be made consumer dummyfied
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Tim Team Reaper.

Could try a kettle approach where when it gets to hot it auto shuts off
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A journey of a million miles begins with a single step followed by a hell of a lot of other steps so get walking

use a nimh charger chip
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Mechanical engineers build weapons, civil engineers build targets

What he means is that for proper charging, the only accurate way to sense a full-state-of-charge in NiCd and NiMh cells is the "delta-peak" method where the cells voltage is monitored after each burst of charge goes in - as long as the voltage keeps increasing, more charge is fed in.

When the battery starts to "spit back" (like a kid thats had to much), and the voltage goes down as you add more charge, the cell is considered full.

This takes a pretty clever charger that can switch the charge on and off in bursts, measure the voltage accurately, remember the last few check voltages and compare the current one against them and finish bursting if it is lower than before.

A less reliable (but still better than no End-of-charge-detection) is to stick a temperature probe in the pack, and if the pack temperature starts exceeding a certain limit, stop charging. This isnt being as nice to the cells, but its better than...

What most wall-wart type chargers do.. just keep stuffing it in slowly. A contant current, or resistor to limit current voltage is fed into the pack. chosen for around a 12-14 hr charge rate (< 1/10 C rating) is fed contantly into the pack and hopefully the user will remember to disconnect them before they've been being slowly overcharged for days or weeks. The #1 cause of voltage-depression (not memory effect) in packs.

If the batteries are oversized enough for the device that they can suffer a 50% drop in capacity in 6 months or less and the device still be useful. (say an electric toothbrush or home handymans drill ) that doesnt need to perform to the last drop of battery charge - a few minutes is enough), then you can get away with bad charging techniques that hurt the batteries a bit each time. If your toothbrush only runs for 10 minutes instead of 15, who cares.

If you a pushing a battery hard trying to get as much energy as possible out of a given weight/size (like we do with bots), then you should use an intelligent charging system so as not waste battery efficiency.

If that and a slow 10 hr charge doesnt matter, then just use a resistor on a suitable voltage pack and ignore the batteries groaning.
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Great minds discuss ideas. Average minds discuss events. Small minds discuss people

What capacity pack is required *just* to power two drill motors for a match?

I am looking at using separate batteries for drive and power; figuring out the smallest size pack for the drive motors isn't as easy as is seems. After looking at motor specs, a commercial 550 size motor produces around 50 watts and R/C car motors go up to 100W (that's "hobby watts" as Spockie would say ).

Using 85 watts as the average power output gives an average current of only 7 amps (85W / 12V = 7.08A). We all know the currents can go WAY higher, but this not a pushy bot we're talking about; I figure that drawing much more than 7A on average will melt most drill motors.

Doubling the amps for two motors and adding a safety factor makes an average of 20A for the drive system. For a 3 minute match, that only works out at 1AH, which seems far too little compared to what most builders are using.

SO, what are your suggestions?

http://www.architeuthis-dux.org/torquecalc.asp

your setup says .9ah which is definitely about right. i can remember draining a single 2.3ah sla in old old old cobra in about 3 minutes and those would derate to about 1ah.

and considering your non pushy driving style id say a 1.2ah lithium would you get you bye just fine.

you could always put the values in edts and drive around as you normally would to get a better value. it was pretty well spot on with my robots

nimh charger chip? Expensive?

Can you buy wall plug chargers 3.6v you could just hook to the pack?
Trying too keep cost down.

Cant say what it is but it may be on the market if all gose well and i dont want mrjb blowing up the battery cos he cant just plug it in and walk off.
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Tim Team Reaper.

http://www.google.com.au/search?hl=en&q=nimh+charger+chip

if you do some of your own work it might make your project go faster.
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Mechanical engineers build weapons, civil engineers build targets

is it possible to solder a standard steel bolt onto the tab of a whyachi switch (not sure if its copper or brass?) or even a brass bolt.

The Whyachi switch tabs are solid copper, so you can solder them easily. I recommend taking the tabs out first to avoid melting the switch.

@Nick:

based on your numbers, 1Ah sounds reasonable. A lot of bots are running with active weapon motors as well on a single string of A123's which are 2.3ah (max).

The biggest variable in your equation is the word "match". No two are the same.

At Robowars 6 - I noted how much power was put back into I.G.'s pack after each fight. The smallest amount was 700mah, the largest was 2100mah, with 1000-1500 for most of them. Thats two drills at 16.8v with big wheels and a weapon motor.

Sounds like the batteries are about spot-on size wise, but thats still a 3x range variability with different matches. Thats with a fair bit of pushing and running the weapon motor about 75% of the time.

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@Tim:

No, you cant just hook a fixed maximum voltage to a NiMh pack. They dont take charge at any reasonable rate until you exceed their terminal voltage by a fair bit. They are charged using a "constant current" (CC) model, and the voltage on the cell *while the power is applied* can be all over the place, so just because it has reached 3.6v doesnt mean its charged.

You have to apply a fair bit more voltage to push current into them to begin with than you actually want them to finish / top out at. So applying a fixed voltage high enough to bring the pack to a full charge will also result in an overcharged pack shortly afterwards if the "full" indications (temperature, neg-delta-peak) are not watched and the charge finished.

Lithiums are easier to charge because they use a CC/CV model, which means you are supposed to limit the maximum current initially - The CC - Constant Current Part - (so you dont try and stuff 50 amps into a flat cell), but once you are past the initial surge of power absorption, all you have to do is hold the end-voltage (the CV - Constant Voltage part) until the current tapers off to below a certain amount, then the cell is full.

The bad part of charging Lithiums is that for multi-cell packs, they really need cell-balancing circuitry. Their full charge voltage characteristics mean that if one cell reaches full before the others, its voltage begins to go up very fast, which can fool the charger into thinking that the entire pack is charged when really its one high-voltage cell and the rest not-yet full.

This means that next use, the undercharged cells will be over-discharged below their minimum voltage (which Lithiums dont like doing much), and the problem repeats and gets worse. Monitoring each cell seperately with a balancer prevents this from happening. So again, a relatively complex charging circuit is needed to be nice to the batteries.

Lead-Acid batteries are about the only common easy-to-charge multi cell battery tech I can think of - but of course they are heavy, and dont like fast or deep discharges. Or high temperatures, or mechanial shock.. If you are flattening a SLA in anything less than 10 hours, you arent going to get full rated capacity from it. In bots where we flatten them in minutes, you're lucky to get 30-40% of the rated capacity which is spec'ed for a 10hr discharge.

Sounds like the usual engineering compromise..

Good,
Fast,
Cheap.

Pick any *2*, you cant have them all in the same design.
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Great minds discuss ideas. Average minds discuss events. Small minds discuss people