For simplicity please keep answers "short and sweet"(funny that is my wifes nickname for ME)
If I discharge my batteries and then go and recharge them the mAh just keep climbing. The 3300's I have will take about 4000 mAh and the 2400 NiCad will take ??? How many mAh should each battery pack take? Is there such a thing as too much? I am charging on a normal 6 amp charge for both types on an Orion Charger.
Help if you can,
RYY
Battery capacities are a measure of the discharge capabilities of the cell type. This means that, under perscribed conditions, your pack is expected to provide a nominal 3300 mAh while discharging. Since we enjoy following the laws of thermodynamics, this means that it will take more than 3300 mAh to charge your pack, since you must put more in than you get out. This excess charge that you put in gets converted to heat as you charge the pack through both resistive heating and an exothermic chemical reaction, which explains where the heat comes from.
Not only that, but many battery chargers aren't calibrated particularly accurately, since they don't really need to be in order to perform their job well. Your charger may indicate that it has put 3000 mAh into your battery, while in fact it may have put only 2600 mAh in (or so). You can confirm the accuracy of your charger by connecting an ammeter in series with your battery as you charge it, and confirm the ammeter reads the same as your battery charger. Don't go by the amount of power the charger is using, since the charger needs power to run it as well.
There is definately such a thing as too much. This was easier to tell with NiCads than NiMH, since you knew NiCads were done charging as soon as they started to warm up. This is because NiCads changed from an endothermic reaction to an exothermic reaction when they reached full charge. My understanding is that NiMH switch from one exothermic reaction to another when they reach full charge, so they go from being fairly warm to hot, which is more difficult to judge by feel.
Over-charging batteries is a Bad Thing (TM), and should be avoided at all costs. If you have a half-decent battery charger configured properly, you shouldn't need to worry about this. You know you have charged your batteries too much when they start oozing, frothing, spewing, and carrying on in a rather obnoxious way.
Quoteis because NiCads changed from an endothermic reaction to an exothermic reaction when they reached full charge
did they actually get cold while charging? Many endothermic reactions do.
Quote from: "haddow"did they actually get cold while charging? Many endothermic reactions do.
Their temperature would drop when you would start charging, yes, but the resistive heating would prevent them from getting cold and overcome the heat sink of the reaction at high charge rates.
Tom,
Charging batteries last night, off the discharger the 3300's took 4800 mAh and shut off with the peak sensitivity. My 2400 Nicads took 2900 mAh. Would you say that was overcharging? Should I set an upper limit on the amount of mAh going into my batteries?
RYY
Like I said previously, just because your charger says it supplied 4800 mAh doesn't mean it really did.
If your batteries are peaking properly, I wouldn't worry about it. Take note of the temperature immediately after they peak, and compare this with someone elses battery pack after they do a similar chare with their charger and their battery pack (by that I mean charge from completely dead like you, and at the same rate). If yours are way hotter, then you're over charging. If the temperatures are similar, then you can strut around and tell everyone else that you can pump more mAh into your batteries than anyone else can.
It's just a number; it's not all that important. It is more useful as a guide to indicate the difference between your packs, and to let you know if anything untoward happened while charging.
i think you have the team orion charger dont you............. the 2400s sound ok the 3300s are takin a bit much but it really depends on temp........ why dont you use the temp probe and set it to 42 i believe thats what koragee suggested.......my 3300s off the tray take about 41-4400 48 is a bit high but as tom has mentioned all chargers are not calibrated to read the same in the mah department or volts as every charger i have seen or atleast used reads different volts than if i used a seperate volt meter(never used a meter to check capacity)but sure it would read different aswell
your quick Tom!
Thanks to both,
Doug I will ensure that my temp probe is set to 42 degrees as you suggested, I think it is set at 47 now. What about a temp setting for a Nicad? Should it be more or less? It always seems that my Nicad charges a bunch hotter than my NiMh!?? Doug after you take you batts. off your discharger do you let them sit a bit before recharging? After twenty four hours, my batts. are cold, onto the charger they are sitting around .5v but if I let them sit for a while they come way up, I think that maybe why my mAh are so high, do you think?
RYY
im not sure about ni-cads temp they do get pretty warm but a correct temp i do not know,as for the tray i put eack pack on for 24 hours most times then they sit till the next times i race so i just put them on about at 6.5 and they take between 41-4400 a cpuple times they hit 42 on temp and shut off.............i would think if you put them straight on charge after being on a tray for a day or 2 it might take a little more mah but probably not too much more than if you put them on a tray for 24 and then let them sit before you charge
A long time ago, a good rule-of-thumb temperature for NiCads was a 20 degree C rise.
Charging by temperature is an excellent method, however it has a few major drawbacks.
1) It doesn't work well on a nearly charged battery, for example, if you are re-peaking a pack that you charged earlier in the day.
2) It doesn't work well outdoors, since sunlight, any sort of breeze, etc. can have a major effect on the temperature independant of the charge state.
3) It doesn't work well if you have a fan around.
4) It doesn't work well if you forget to attach the temperature probe.
I would expect a suitable NiMH cutoff temperature would be higher than a suitable NiCad temperature, since NiMH (as mentioned previously) tend to get hotter than NiCads while charging.
If you deep-cycle your packs every charge, be warned that this tends to reduce the cycle life of your pack. It will also explain why your packs take so much of a charge.
Note that a no-load terminal voltage is pretty meaningless. I can take a multimeter and, after walking across carpet, measure several thousand volts between me and a doorknob. That doesn't mean that if you plug me into a race car I can make it move a measurable amount. A dead pack, after being removed from a deep cycle tray, will stabalize with a nominal voltage of in the neighborhood of a volt a cell, given enough time, even though there is no appriciable charge in the battery.
mAh rating is typically rated from a full charge to a voltage of 0.9V for NiMH (I think NiCD is 0.7V but don't quote me). Here's how it works. You can discharge a pack of 3300mAh batteries at 1C ( 1 x 3300mAh) or 3.3A. The pack will discharge at this rate for one hour before the voltage per cell drops to 0.9V. If you charge your battery once it reaches 0.9V you will get typically 3300mAh back into the battery. If you are wise with your batteries you use a discharge tray and bring them well below the 0.9V cutoff. You are taking more energy out of the cells than the rating indicates because you go past 0.9V bringing them down to near zero volts.
When you take a battery that is near zero volts off a discharge tray you will notice the voltage rises to around 1.2V. This is a good sign that the battery is healthy. It will not provide that voltage under load but it is a good indication of the "floating" voltage of the cell. If you put any sort of a load on the cell the voltage will quickly drop.
In order to fully recharge a battery that has been fully discharged you must put in x mAh to bring the cell up to the 0.9V "discharge" state and the 3300 mAh (or whatever mAh rating the cells have) for a complete charge. That is why you are getting 4600 mAh going into the battery. A small ammount is lost in heat, some is needed to get it back to its 0.9V and usually more than the 3300 mAh rating is needed to bring it up to a full charge.
When charging NiCD you can use the thermal method however you typically need a fairly controlled environment. For NiMH delta peak detection works best. What happens is the voltage on the cell continues to rise until the cell is almost charged. The batteries don't heat up a lot at this point. Once the battery has reached near its full charge the voltage does not continue to rise. The battery will also start to heat more at this stage as more of the electrical energy is converted to heat. Now the voltage will start to drop since the cell is very near its full capacity. This is where the charger circuitry detects a small DROP in voltage (even though its still pumping electricity into the battery. This is typically measured in mV - I think the SMC batteries we use with the club are around 9mV - check on the label. Battery heating is at its highest near this voltage.
Now heat and batteries is a love/hate relationship. You want to re-peak your batteries before you go out and race. Some people think it is to get the most capacity out the battery but typically you are only putting in a few hundred mAh most of which is converted to heat. What you are doing is bringing the temperature of the battery up which helps keep the voltage and current high because of the chemical reaction in the batteries. I've heard 50C is optimal for racing however this cuts into the life of your batteries - heat is its number one enemy. I just let it peak itself and the battery is nice and toasty and ready to rip. Some chargers will let you do multiple repeaks with a delay between to heat the batteries more. Not to sweat it though a few laps and the battery is usually up to temp.
my charger has delta peak on it what should it be set at the manual says 8 mv/c for nimh and 6 mv/c for nicads depending on charge rates.
what about charge rates? 6 amps seems really high, but i heard that higher amps gives you better voltage, is there any downside besides higher heat generated and less accurate peak? whats the best charge rate?
That sounds suspicious to me. When NiMH first came out, there was quite a bit of worry since they tended to not peak as distinctively as NiCads, and they tended to get quite a bit hotter while charging as well.
The general conclusion was that they are safe to peak charge, provided you use a sufficiently sensitive peak detection method with thresholds much lower than that used for NiCads.
I question your manual reccommending a higher peak threshold for NiMH than for NiCad.
In the good old days, a good rule of thumb was 10 mV per cell for NiCad, and I've seen as high as 8 mV per cell reccommended by battery matchers (not manufacturers) for GP NiMH cells, but when NiMH first became popular, cutoffs closer to 2 mV per cell were more common.
The biggest problem with overcharging is due to the high temperatures generated. When you're charging a dead battery pack, the majority of the energy is used to drive a chemical reaction which stores the electrical energy chemically. When this reaction has completed, the electrical energy is then used to generate hydrogen in the cell, which increases the cell pressure. If you keep on overcharging, the cell will first vent, but if the vents are blocked (which often happens with solder or flux or other reasons), the cell will first bulge, and perhaps explode. If you set too high a cutoff for your peak charge, most likely you won't overcharge that extensively, so the problem will mostly be limited to high temperatures in the cell. These high temperatures can contribute to drying of the electrolyte (as hydrogen gas is generated from the electrolyte, which can seep out of the vents especially if they have been damaged from high temperatures caused by soldering).
The best charge rate depends on lots of things, mostly on your racing budget. As can be concluded from above, higher charge rates will tend to shorten the life of your pack. I've seen it speculated here that the real reason for improved performance from a high charge rate comes from the increased cell temperature, which results in a faster chemical reaction, which would suggest any high charge rate benefit is short-lived. This is by far the best explination that I have heard, and this would suggest there would be no significant difference in performance if the packs are allowed to cool after any current is used to charge your pack.
However, the charge rate is said to have an effect on the shapes of the crystals that form that contribute to the chemical reaction that gives rise to the voltage in the cells, and that trickle charging tends to form larger crystals, which tends to reduce the surface area of the reactants, slowing the chemical reaction (and hence electrical power sourced) in the cell. However, these currents are by definition far lower than what anyone charges at with a peak charger.
So, yes, there are downsides to higher charge rates, in that your chances of turning your expensive pack into a cherry bomb are significantly higher for a minimal, short-lived boost in performance. It seems as though 6 amps is a good compramise between maximum performance and reasonable safety and cell life, although if you want your packs to last longer, you would definately benefit from a lower charge rate, especially if you typically crash more than once during a race.
A single crash will cost you far more in a race than your battery charging technique.
ok thank you makes sense
So, Ill lower my delta peak and experiment.
My charger is limited to 3 amps when plugged to the wall and 5 amps connected to 12v source.
So, I have some new 3300's. The charger wants me to enter the capacity, do I set it at 3300 or higher? 3300 being the total being delivered to the pack? but the previous post says his took a LOT more, but being charged from totally discharged state. Does my charger take into account the energy delivered wasted as heat?
I would expect you should enter the rating of your battery, which is the discharge rating, or 3300 mAh. The fact that it takes more to charge a battery than it supplies while discharging is covered by one of the laws of thermodynamics, and thus is present in all battery types.
In short, don't worry about it and try it. You know if your battery charged correctly if you feel the temperature immediately after it finishes charging. It will be quite warm; if it's not, then that means it didn't charge all the way.
The mAh rating is based on the voltage cut off of 0.9V - if you use a discharge tray you go beyond that limit. Set your battery charger to 4500 mAh if you are using a discharge tray or 3800 if you are going to just discharge to 0.9V per cell.
Start with 8 mV per cell for GP3300 cells... also use the top off feature most newer chargers have for NiMH batteries. This will perform the same function to equalize the charge on the cells with a low charge rate. 5 or 6 amps is a good charge rate for life/performance of the GP3300 cells.