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After a lot of research and experimentation I have come to learn that the sentence "This is a 1.5V, 2800mAh battery" is entirely a lie.

(i.e., the potential difference between the terminals of a battery changes over time and the shape of the graph is dependent on battery chemistry, ambient temperature and current draw, as is the useful energy capacity. Finally, many things which are colloquially called "batteries" are actually cells, and this is important as it has a bearing on both of the above.)

What I want to know is, if I have a 12V battery of lithium cells whose spec sheet says it will deliver 9A and I attempt to draw 9.5A from it, am I heading for a quick trip to the burns unit? What about NiMH or other chemistries?

How much headroom ought I leave and what are the factors (temperature, "C" rating, duty cycle, anything else) I need to consider?

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  • \$\begingroup\$ I think the headroom is already taken into account, and that you shouldn't exceed the given value. \$\endgroup\$
    – stevenvh
    Jul 17 '11 at 14:50
  • \$\begingroup\$ Shouldn't exceed it ever? Shouldn't exceed it for more than 500ms? Shouldn't exceed it over 40ºC ambient temperature? And what will happen if I inadvertently do? Part of the problem is I don't actually know how much current my circuit is going to draw under all the above variables. \$\endgroup\$ Jul 17 '11 at 14:56
  • \$\begingroup\$ Is it safe to exceed by 1mA? Yes, Absolutely. 100A? No. So it's between these two. No-one will tell you where the actual limit is, also because that limit will vary with conditions. How far do you dare to go? What's it worth to you? If you don't want to take risks, stick with the specs. If you're a high-risk man, the sky is the limit. \$\endgroup\$
    – stevenvh
    Jul 17 '11 at 15:05
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    \$\begingroup\$ It seems you need to look up what "maximum" means. \$\endgroup\$ Jul 17 '11 at 16:05
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Good question. Big question. Partial answer ...

Reputable manufacturers provide specification sheets (yes, even for batteries) and these will provide recommended maximum continuous currents and may provide peak allowable discharge currents.

The maximum value is NOT a hard and fast limit which may not be exceeded, and how much it matters depends on the battery chemistry, the specific implementation and on how much you care about the result. Slight but continuous over current discharge may led to reduced cycle life at a rate disproportionately high compared to the amount of over discharge.

There are many Li (Lithium) chemistry based systems. Some are primary (non rechargeable), and some secondary (rechargeable).

Starting with LiIon (Lithium Ion) which is probably what you meant. These are the most common Li secondary cells available and have related "spinoffs' such as LiPo (Lithium Polymer). They have close cousins in eg Lithium Ferro Phosphate (LiFePO4) whch is a lower capacity but MUCH better behaved variant.

LiIon have a charming "feature" known euphemistically as "vent with flame" (VWF) (to which you can append :-) :-( !!!! ) ie when used in modes outside spec (or sometime just because they can) they will self destroy with heat flame smoke and general hilarity.

LiIon are generally rated at 1C max charge rate and 1C to 2C max discharge rate depending on manufacturer, model etc. Exceeding the max recommended discharge rate modestly is not liable to cause problems. 10% or 20% is probably OK and maybe even 50% or 100% MAY be OK . YMMV and you can have no complaint if it does VWF.

Charging LiIon above their specified rate is a really bad idea [tm]. As above, it may work OK but certainly may result in "vent with flame". Again, I'd hazard that 10% or 20% is liable to be fine and maybe double may be OK. Or not.

If you use LiIon at rates beyond rated values you will generally degrade their cycle life by accelerated amounts. eg I'd guess that a consistent 20% overcharge may halve cycle life. Informed guess only. Similarly, by running LiIon at somewhat below spec the cycle life can be usefully extended.

LiIon also have a very tightly specified upper charge voltage - usually 4.2V with some variation specified across temperature. Exceeding this by 0.1 volt is "unwise" and by 0.2 v is very very unwise. eg 4.2V std, 4.3V hazardous, 4.4V stupid. BUT lowering the max charge voltage slightly to say 4.1V or 4.0V will greatly improve the cycle life and also lower the charge capacity. eg 4.1V max charge voltage may be 80% - 90% of capacity.

At the bottom end, lifetime is also affected by Vmin. There is very little energy left below about 3.0V* and stopping discharge at 3V or even above can be a very good idea for lifetime purposes. (* Discharge curves not to hand - look at manufacturer's graphs. Note that voltage depends heavily on load. Heavy load will drop acceptably lower than light load.

There are numerous "new" versions of liIon being announced regularly. Few have yet got to market. These may have charge or discharge rates of 10C or even 100C. ie at the top end of claims, charging in under 1 minute is claimed.

Lithium Polymer (LiPo - NOT to be confused with LFP / LiFePo) uses "plastic" materials for electrolyte retention and generally have somewhat superior electrical characteristics and somewhat greater resistance to VWF destruction. Somewhat.

A very worthwhile variant of LiIon is LiFePO4 / Lithium FerroPhosphate. Sometimes referred to as LiFe which is OK enough as long as this is not taken to be the chemistry. I'll use LFP. LFP allows charging at 1C to 2C (some manufacturers 0.5C) but discharging at 10C or more (some eg 30C) Energy contant is low wrt LiIon (about 60% or less) but cycle life is vastly superior and performance at high and low temperatures may be superior. Properly managed LFP offers 2000 deep discharge cycles (against 300-500 for LiIon) and vastly greater figures are claimed by some for larger batteries with good management.

As always - see spec sheets. Top manufacturers provide a large amount of information re rates, voltages, cycle life etc.

LiIon is a joy to manage charge wise.

NimH (see below) is an ornery pig. You can get OK results with NimH using simple methods but best results need rocket science or necromancy.

Reputable manufacturers equip LiIon cells with internal protection circuitry. When a single cell MUST have electronics inside to make it half safe you know you have a fun product. Very low voltage LiIon need to be coaxed into normal range with great care. Very very low LiIon are usually declared dead by their controllers. Insisting on charging such (bypass protection( may result in death (usually just the cell) but can work with due care. People make special bags for charging liIon cells in. What does this tell you?

All that said, an excellent technology. Treat with due care.


Lightly:

NimH: Charging up to 1C Ok with monitoing of negative delta V or delta temperature or absolute temperature for termination. Some allow 2C with speial batteries. Smart monitoring may allow 2C+ with care. Radio control model fans charge NimH at 4C or more using capacity x 1xx% overcharge as charge termination. eg they may charge a 4Ah pack at 20 A for 15 minutes. This is 5C and 125% energy input. Lifetimes suffer. They don't care.

NimH may be more or less discharged at whatever rate they will bear. Internal cell resistance drops voltage increasingly at high current making battery less useful unless designed accordingly. Discharge should be stopped at say 1V at lowish loads and no less than say 0.9V at very high loads. I'd err on the high side. You can discharge them to utterly empty (0.8-0.9V ) but you gain little and will very severely impact lifetimes. NimH is good for 300-500 deep discharge cycles but can be taken to 2000 or so by taking 10% off top and bottom (stop discharge early, terminate charge early).

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  • \$\begingroup\$ Most lithium batteries of any significant size have built-in safety features to prevent vent-with-flame, though in many cases once these safety features have been activated the battery pack will be rendered permanently useless. \$\endgroup\$
    – supercat
    Jul 17 '11 at 16:12
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If a battery is specified to deliver 9 amps, and you limit current to nine amps, the battery will likely achieve lifetime performance reasonably similar to what is specified in the datahseet. Going beyond the rated current may not cause immediate failure, but is likely to adversely affect device lifetime. Trying to draw e.g. 10 amps from a 9-amp battery might, depending upon circumstances, have no meaningful effect on useful life, or it might cut its useful life in half (e.g. if the battery is operated at the top of its temperature range, it may be significantly damaged by currents which would pose no problems at lower temperatures).

Note that if excessive current will be drawn for an interval measured in whole seconds or minutes, there may be thresholds near which even a small change in current may have a huge effect on battery lifetime. For example, if beyond a certain temperature internal resistance increases significantly, then drawing an amount of current which would keep the temperature below that point may not overly affect battery life, but drawing enough current to reach that point may create a 'partial thermal runaway', where the battery ends up establishing an equilibrium temperature much higher than what it had at a lower current. The thermal runaway won't necessarily go far enough to turn the battery into a molten slagheap (it's possible the increasing internal resistance may end up limiting the current enough to prevent that) but may very well go far enough to seriously degrade the battery's useful life.

Hypothetical example, chosen for numerical simplicity rather than realism: Assume the resistance of a 100-volt battery will be 0.01 ohm at 50C and doesn't go below that, and every twenty-degree change in temperature will represent a tenfold change in resistance, and power dissipation is one watt/degC for a device operated at 25C. If the battery is at 50C, one could draw 50 amps through the battery all day (with a 0.5 volt drop, the battery would generate and dissipate 25 watts of heat.). Attaching a 50.01-amp constant-current load would cause the battery temperature to start rising, and with the rising temperature causing both resistance and power dissipation to start increasing, leading to a thermal runaway.

This process would be limited, though; for the temperature to reach 130C, where its resistance would be 100 ohms, the device would have to be dissipating 105 watts. At 100 ohms, though, even with a dead-short load, only one amp could flow out of the battery, thus causing no more than 100 watts to be dissipated in the battery. Thus, provided that a temperature of 130C wasn't so hot as to cause an internal short, the battery wouldn't melt down. On the other hand, the battery would be operating with an internal temperature far above normal temperatures, and life could be greatly reduced.

Note: real batteries may or may not establish an upper temperature equilibrium before really nasty things happen (internal shorting, explosions, etc.) but even when they do establish such an equilibrium, the stress it would put on the battery would be likely to destroy it in a short time. Note too that even if the internal resistance of a battery that was heated uniformly would reach a "safe" condition, it's possible that extreme overload conditions may cause part of the battery to reach a "disaster" temperature before the battery as a whole has reached the safe equilibrium. One may expect that an overloaded battery would safely turn into a benign slagheap, but that doesn't guarantee it's not going to explode instead.

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