Battery performance question ?

[curry87]

Ive ran some tests with two identically 200mah 8.4v nimh pp3 batterys both being fully charged and then discharged right down to 7.2v before being disconnected from the load automatically and then reconnected after 30 min of battery recover time.The load is simply a resistor of value 100ohms.

Call the two batterys A and B this is result i got:


battery A
after 41mins voltage dropped to 7.2v
load disconnected for 30mins and then reconnected
after 30mins,15sec voltage dropped again to 7.2v
load disconnected for 30mins and then reconnected
after 11mins voltage dropped again to 7.2v




battery B
after 36mins voltage dropped to 7.2v
load disconnected for 30mins and then reconnected
after 30mins,54sec voltage dropped again to 7.2v
load disconnected for 30mins and then reconnected
after 4mins voltage dropped again to 7.2v


First question how can the battery recover to such an extent to still retain a large amount of capacity ?


Second question how can you be sure the battery is fully discharged if it behaves like this where it recovers to this extent?


Third questions why does the battery performance of both fall short of the 200mah capacity ?

 

[Sceadwian]

The cells were obviously not identical.. Manufacturer variance for starters. Battery capacity is always rated as a fraction of the capacity for discharge. lead acid batteries are a good example as they're typically rated at a 1/20C discharge rate, meaning if you discharge a 20AH lead acid at 1amp you'll get a 20AH rate from it, if you discharge faster than 1/20th of the capacity the heating and internal balancing currents will eat some of the power.

NiMH's are derated differently for discharge current, but the same rules apply the curves are all that's different.

I'm not sure of the standards (if there are any) but any major maker of batteries will show their stated capacity value rated for discharge current in the PDF for the battery.

Deep recovery like you're experiencing is related to bulk heating effects. The voltage drop over time is not linear, in order to properly determine the capacity you'd need detailed graphs of the discharge curves. Your discharge current is 1/3 of it's rated capacity, this is why the AH rating is lower than the listed 200mah, because the 200mah listed is at a lower discharge current than listed. I'm going to hazard a guess that typical AH capacities listed on NiMH's are around 1/10th C, maybe 1/5th, but no higher.

The differences in the two batteries seems more than I would consider reasonable for like cells. When you say identical do you mean fresh brand new cells off the line with sequential serial numbers or just two random batteries with the same label on them? Are you sure of the history of the cells? Cells age.

 

[RCinFLA]

Discharging to 7.2 vdc is only discharging a cell to 1.20 vdc which at the current you are discharging is only about 50% of capacity of cell. The recovery should only be about 50 mV per cell so the no load voltage after you first discharge cycle was in the range of 7.4 vdc.

Discharging to 1.1 vdc per cell (6.6 vdc) will get you down to about 10% to 15% state of charge at the current you are discharging. The voltage will slump down pretty quickly from 1.1 vdc.

 

[audioguru]

You are killing those tiny Ni-MH batteries with such a high current (about 90mA). Use a max current of 40mA instead so the voltage will drop to 7V in about 5 hours.

 

[curry87]

The differences in the two batteries seems more than I would consider reasonable for like cells. When you say identical do you mean fresh brand new cells off the line with sequential serial numbers or just two random batteries with the same label on them? Are you sure of the history of the cells? Cells age.

Same manufacturer "HI-WATT" and spec both fully charged and same age.


Why are there discharge curves for a similar battery that are of 1C if 0.2C is the maximum safe discharge rate ?

 

[RCinFLA]

Same manufacturer "HI-WATT" and spec both fully charged and same age.


Why are there discharge curves for a similar battery that are of 1C if 0.2C is the maximum safe discharge rate ?

That is a seven cell package. Some 9v rechargables have six cells, some have seven cells. You can discharge NiMh at 0.5 to 1C rate.

 

[Sceadwian]

Audioguru was being excessively dramatic about the effects of discharge rate on cell life, you are definitely not killing the batteries, though a lower discharge current will always result in a longer cell life, especially in battery packs (not just single cells) RCinFLA's value of .5 to 1C sounds perfectly reasonable to me.

A ran for approximately 82 minutes, and B ran for 70, that's a ~13percent run time difference, again that seems high for manufacturer variance, right up until I looked up what Hi-watt is, which is a Hong Kong based battery maker, for cheap made in china batteries +/- 20% capacity difference between similar packs wouldn't surprise me in the slightest. If you want a multi cell battery pack to last any length of time you have to use high quality well matched (same capacity) cells. The cells you're using are going to invariably lead to an early pack death significantly faster than if it'd been made from quality cells.

 

[audioguru]

Audioguru was being excessively dramatic about the effects of discharge rate on cell life, you are definitely not killing the batteries, though a lower discharge current will always result in a longer cell life, especially in battery packs (not just single cells) RCinFLA's value of .5 to 1C sounds perfectly reasonable to me.

I looked at the datasheet of the 9V Ni-MH battery sold by Energizer (American). Their battery is made for them in Germany. It works well up to a current of 0.2C but the curve at 1C drops its voltage like a rock. I don't believed what is said in a Chinese datasheet.

 

[Sceadwian]

Energizer is an American brand name, Energizer hasn't made batteries here in the states in many years.

I have no idea what possible pertinence this has considering the Hi-watt brand the OP mentioned is a Chinese branded junk brand. Even the German made units you're talking about have no relation to this post.

 

[curry87]

Whats a good make manufacturer of theses batteries in the uk that are not cheap chinese rubbish ?

 

[audioguru]

Energizer is an American brand name, Energizer hasn't made batteries here in the states in many years.

I have no idea what possible pertinence this has considering the Hi-watt brand the OP mentioned is a Chinese branded junk brand. Even the German made units you're talking about have no relation to this post.

Energizer has alkaline battery cells made for them in the USA. Their cylinrical Ni-MH cells are made for them in Japan, maybe by Sanyo because they have the same spec's (the newest ones hold their charge for a long time). Their old 6-cells Ni-MH 9V battery was made for them in Japan but their newest 7-cells one is made for them in Germany.

I have never bought a cheap Chinese battery in the UK so I just talk about the batteries that I use. The high quality German 9V Ni-MH batteries that I use do not perform well above a discharge current of 0.2C, I think because each cell is tiny.

 

[Sceadwian]

My first instinct is to say Sanyo, they've been an industry leader in rechargeable cell technology up till modern Lithiums where they have massive competition, the bulk majority of quality cells you see sold by other companies are actually redistributed Sanyo cells.