im trying to work out capacity of batterys, but ive realized just putting a fixed load on them wont work because the voltage reduces then current changes, just wondering how to do it is there a piece of equipment that one needs available?, suppose it would monitor the voltage and change the load to suite maby
cheers
Battery capacity is measured in units of charge, such as Ah or coulombs, not energy.
You can multiply this by the nominal voltage to get capacity in units of energy if
you want.
If you want to measure the true energy capacity under a specific discharge
regime then you do need to monitor both voltage and current for the whole
discharge and integrate their product.
Often battery datasheets have a few "typical discharge" curves in them, which
show the change of voltage and charge capacity under load.
i think ive found what i need a constant current dummy load, as voltage changes it changes the load, perfect! my interest is measuring the capacity of Li-lion 3.7V 18650 batterys
cheers
a constant current dummy load,
You got a link to one of those?
there you go mike there are a few on you tube, dave jones also has built one and got a vid on one, im going to build one if you want a pcb
Hobbyking sell a range of chargers primarily for charging batteries for RC models, but you can use them as general purpose
battery chargers for Lipo,NIMH,Nicad and Lead Acid.
They can charge and discharge at a fixed rate which is user presettable, so are useful for measuring a batteries capacity as they have a timer , and you can program them to stop charging and discharging when a given voltage is reached.
When testing rechargeable batteries, do make sure to set a threshold voltage below which you disconnect the load. For many (most) battery chemistries, if you discharge them too deeply then you risk irreversibly damaging the pack. For example, NiCd and NiMH cells are nominally 1.2V/cell, but never, ever discharge series packs to less than 1.0V/cell. Lead-acid packs should not be discharged below around 1.1V/cell. There are similar limits with Lithium batteries too, and the UN compliant ones have inbuilt protection circuits which disconnect the battery if you discharge them too much.