Basic PHEV Light, battery cooling questions

[crx_rogus]

Folks -
I've read though the Multiple Subpacks Balancer thread, much of the MIMA discussion (esp. on 99mpg.com), the main PHEV thread and the BEV -grade BMS engineering opus magnum retepsnikrep has done, and have some relatively simple-minded questions regarding nightly on-the-cheap charging of the factory battery in the car with the car off, and one regarding battery cooling:

1) How long before subpack destruction typically starts if feeding current-limited full charge voltage to the ends of the battery pack with the car off?

2) If using a relay array to cycle a charger to each subpack one subpack at a time (again with the car off), how long would it take for the batteries to develop a problem with being charged in pulses of sizable current 1/20th of the time vs. a continuous, lower steady current?

3) What mpg improvements and/or error codes being thrown have folks found with regular use of either or both of the above?

4) To minimize the formation of battery imbalance issues while driving, has anyone found an additional fan or two squeezed inside the battery compartment to help via minimizing hotspots within the battery pack where cool air doesn't reach as well?

If those questions were already answered many times already, I apologize but have scoured the forums a fair amount now and have not yet noticed answers to those questions.

TIA,
Roger

[IamIan]

Originally Posted by crx_rogus

Folks -
1) How long before subpack destruction typically starts if feeding current-limited full charge voltage to the ends of the battery pack with the car off?

not as straight forward a question as it seems.

The easy answer before significant / major / permanent damage:
Anything under ~20 mA can be left on indefinitely.
Anything under ~65 mA could be left on for about a month.
Anything under ~130 mA could be left on for about a week.
Anything under ~250 mA could be left on for about 48 hours.


More accurately ...

The flow of charge current is never truly evenly distributed inside a battery cell... even if you put a single cell on a dedicated charger.

Electricity will find the path of least resistance not the path of lowest SoC.

The chemical reactions inside the battery that convert the electrical energy to chemical potential energy are much slower than the speed at which electrons flow / move.

The faster you charge or discharge a battery cell ... or the higher the current used ... the more profound the above situations become.

NiMH cells as a chemistry type are tolerant / forgiving of abuse... so even if you over charge them they don't just die right off... instead you end up doing small bits of permanent damage... over time these small bits add up...

The reason the service life of a NiMH battery is greatly extended when you avoid fully charging or fully discharging it... is that fully charging it almost always slightly over charges some part of the cell ... and fully discharging it almost always slightly over discharges some part of the cell... if done properly that some part can be as small as just a couple of molecules each time...

Because of the above there is no truly 100% safe level... any level does / risks some amount of damage... depending on the specifics ... such as initial SoC... Ambient Temperature .... Cell Capacity ... method of charge termination ... etc...etc... will determine the degree of damage , and weather it is enough to be significant or not.

It is more accurate to think of it as a exponential slope ... it almost never reaches 0% damage not matter what you do.... but you can greatly reduce how much damage is done even if you just follow some more simple things... that is why the trickle charge option is attractive to so many people... it is pretty simple to have a low current level ... and for as simple as it is... is does a pretty good job over all in reducing potential damage.

Originally Posted by crx_rogus

2) If using a relay array to cycle a charger to each subpack one subpack at a time (again with the car off), how long would it take for the batteries to develop a problem with being charged in pulses of sizable current 1/20th of the time vs. a continuous, lower steady current?

see above... there is no 100% accurate simple answer...

In general the relay array cycle follows the same rules as #1... but sense getting a charger to do the more advanced types of charge termination techniques gets harder and more complex... it is easier if you can buy a good charger already with these features built in... but the chargers with the more advanced features built in are much less expensive when you buy types that are mass produced... so you buy one used for smaller battery packs that is more mass produced... it is cheaper and easier for you to get those more advanced techniques... but sense this mass produced charge can't handle 120 cells all at once... you have to cycle it through the relay type system.

Originally Posted by crx_rogus

3) What mpg improvements and/or error codes being thrown have folks found with regular use of either or both of the above?

reads more like 3a and 3b.

3a) Depends on how you will be using the charge you put into the cells.

If you are doing it as a means of battery maintenance than it is a way to try and avoid battery replacement and has nearly very little effect on MPG.

If you use it to top of the stock IMA pack... you are avoiding a bit of gasoline usage to top off the pack... how much gasoline you avoid will depend on how many Ah you put in... and the situations specifics... such as what kind of MPG do you already get?

3b) error codes are wide and varied and depend greatly on the specifics of how you do it... it can be from no error code... to complete IMA destruction.... or anywhere in between.

Originally Posted by crx_rogus

4) To minimize the formation of battery imbalance issues while driving, has anyone found an additional fan or two squeezed inside the battery compartment to help via minimizing hotspots within the battery pack where cool air doesn't reach as well?

The additional fans could help some but exactly how much??? that would be difficult to determine.

Remember the uneven heat distribution is not the only reason a multi-celled battery pack with develop imbalances... I suspect it is a significant one... but there are others as well.

[retepsnikrep]

Ian's answered a lot of this but my two penny worth.

1) As long as the heat doesn't rise too much I suspect you can trickle charge at 250ma or so for a week or more without issue. I've certainly done 2-3 days.

2) No problem as long as they receive the same, this is the basis for my multiple subpack balancer project. (schematic now started)

3) I never had any errors with charging standard pack at 250ma or so, Meant you started off with a full pack each day, so say 5-10% better mpg on shorter journeys.

4) Never worried about or tried that, I don't believe it's a major issue.

Peter

[uhtrinity]

I would also add:

It depends on the environment. With a fan running and a low enough temperature the batteries will be unable to retain heat. The opposite is true if it is hot outside. with 90 degree temp and the car sitting in the sun I have seen charges as low as 200mA make a pack reach over 110 degrees F with a fan running. The batteries heat slowly and loose their heat slowly, even under high amps in / out due to their mass.

[Mike Dabrowski 2000]

If the cells get over 125- 135F, the internal pressure can vent the cells. This is where the damage takes place. Monitor the pack temp while charging at 300 MA, fan running on 12V, and stop when the temp gets over that, and you should be able to charge nightly with little effect on battery life.
A timer that can be set based on the SOC when you start is another simple termination approach.At 300 ma, little heat is generated until the pack is at 100%SOC
Of course this is only an opinion, so proceed at your own risk.