[last updated: 2025-02-16]
Li-ion batteries
battery projects
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On This Page:
- Battery C-rating
- Coin cell batteries
- NiMH batteries
- multiple-battery packs (usu. LiPo)
- measure battery internal resistance
- Misc notes
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- Battery C-rating:
The "C-rating" of a battery (at least usually but not always???) is its A-hr capacity divided by one hr (ie. same number, with units converted from A-hr to amps)
For example, a 2 A-hr battery will have a C rating of 2 amps.
When you specify what current (ie. amps) a given battery type (ie. its specific chemistry) can be charged or discharged with, you'll do so in multiples of C.
For example, a ... battery might be charged at 0.5C, and discharged at 0.1C. This means, say for a 2 A-hr battery (with C = 2a), you can charge it at a rate of 1a, and should discharge it at or below 0.2a.
Charging faster than recommended can overheat and explode. Discharging faster than recommended may also overheat, but mostly will damage the battery's future A-hr holding capacity.
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- Coin Cells:
- Alkaline:
- Coin cells with ID's that start with 'L' are alkaline cells, with a nominal voltage of 1.5V.
- Alkaline batteries like LR1154 (aka LR44) is 11mm diam and 5.4mm thick.
- Lithium:
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- "standard" NiMH rechargable batteries are ~ 1.5v
lifespan: ~ 1000 cycles
heavier, lower power capacity
lower discharge rates
best to charge with a pulse charger
charge at 1.5C
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- Making packs of multiple batteries for higher voltage or capacity:
- If you have a battery pack made up of some number of batteries in parallel,
and you charge it (ie. all batteries in the pack are getting charged at the same time),
then the amount of charge each battery receives will depend on its internal resistance.
- If some batteries in your pack are eg. much lower resistance than other batteries,
then they will end up being more fully charged than other batteries in your pack that have higher internal resistance.
- In order to keep your battery pack balanced, with all batteries about the same charge,
all batteries in your pack should have about the same internal resistance.
- If your pack is not balanced, you can help the situation by breaking apart the pack and charging each battery by itself. Of course this might not be convenient.
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- Measuring battery internal resistance:
- For starters, measuring DC resistance as below is only a reference point
that can be used to compare batteries, or compare how a battery ages over time and use.
DC resistance ignores effects of internal or external circuit capacitance and inductance and frequency.
- Vbu = battery voltage unloaded
Vbl = battery voltage under load
Rl = load resistance
Rb = battery internal resistance
then:
Rb = (Vbu – Vbl) * Rl / Vbu
- Another method uses a small AC signal...
- Internal resistance of a lead-acid 12v battery should be on the order of 10 - 50 milli-ohms.
As a battery ages/degrades, its internal resistance increases.
- See also: (link to:) Battery University
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- Misc Notes:
- Max charge current (amps) for lead-acid batteries is 25% of AHr capacity
- AH rating: determine how long it takes to discharge to ~50 percent capacity.
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