my radio states that i should only put 1800mAh batteries in it. i have some 2450 mAh batteries. can i use these? as i understand, milliamps are more the measure of the lenghth of life per charge than of current output; so i think they'd be ok. i don't want to risk my radio, so i thought i'd ask your opinion.
do milliamps matter???
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Manufacturers only recommend equivalent spec batteries in devices that charge.
In general, your understanding is correct. Volts are what drive products (too few, no go; too many, uh oh!). Milliamp-hour capacity determines how long the product will operate (or take to recharge).
In general, your understanding is correct. Volts are what drive products (too few, no go; too many, uh oh!). Milliamp-hour capacity determines how long the product will operate (or take to recharge).
so perhaps i don't want to charge the 2450's in the scanner, but it's probably ok to use them?
It will just take 50% longer to charge them (assuming that there is not a timer cutoff, in which case it might not fully charge them). Lower capacity than design is going to be something to definitely avoid, though.
well, the scanner has a timer on the charger, which was my first indication that something was not going to go as expected when charging. i've never heard of setting a timer, most charging is plug it in and it turns off when finished. hey, what do you mean by: Lower capacity than design is going to be something to definitely avoid. BTW, thanks for your input and help!!
Products designed to charge NiCd or NiMH batteries internally set the charge rate to no more than about C/10 (total capacity/10) for the batteries included with the product. Batteries can typically dissipate most overcharge as heat at this rate, only minimally affecting overall battery life if batteries are charged beyond their capacity. If you use lower-capacity batteries, then the charge rate will be too high to allow the excess charge to be neatly dissipated, and could result in more rapid battery failure.
For example, if the device is rated for 2800 mAH batteries, it will set the charge current around 280 mA, which the included batteries can handle, even in overcharge. But, if you replace them with 1400 mAH batteries, they will still charge at 280 mA...which will be a C/5 rate for those batteries, and this much overcurrent can rapidly damage the batteries.
For example, if the device is rated for 2800 mAH batteries, it will set the charge current around 280 mA, which the included batteries can handle, even in overcharge. But, if you replace them with 1400 mAH batteries, they will still charge at 280 mA...which will be a C/5 rate for those batteries, and this much overcurrent can rapidly damage the batteries.
so if i want my 1800mAh batts to last (that came with the scanner), don't put them in the external charger that came designed to charge 2450 mAh batts? however it's ok to charge the 2450 batteries in the scanner? i'll just need to get the charging time worked out?
most of us who want to ensure our batteries are fully charged for maximum runtime in the scanner use an external charger, like the Maha Powerex Wizard One MH-C9000. We'll keep a few sets of 3 nimh batteries that we cycle through. put a fully charged set in the scanner in the morning. put the batteries that come out of the scanner in the charger, which will charge them to the optimum level, absolutely no guesswork. Run the scanner during the day, at bedtime, plug the scanner into the wall power supply but keep the battery type switch set to alkaline so they don't charge. We'll charge them in the morning when we swap in a freshly charged set from the Maha. Repeat the process each day, sometimes 2x in a day depending how long we're away from home.
We use external smart chargers for at least 2 reasons.
1- you don't risk overcharging the batteries in the scanner, which is a significant possibility if you have a long charge timer set on the scanner. say you let the scanner charge for 16 hours and its nearly done, then you unplug it to go out and water the garden or something, then come back in and plug the scanner back in for the night, get ready for alot of overcharging, because its going to charge for 16 more hours and might only need 1. the scanner has a predefined charge current that doesn't change based on battery capacity or the battery's need to be charged or not. As upman said, capacity divided by 10 should be a safe current to overcharge a battery without melting it down and damaging your scanner, but it WILL reduce battery life over repeated exposures.
and 2- its better for the batteries to be charged slowly and to just the right level. Smart chargers like the mh-c9000 stop the charging at the right point. A timer charger in the scanner does not.
Now I understand that uniden couldn't put a charger as advanced as the maha in their scanner, and the fact that they put a settable charge time setting in the scanner is proof that they understand that overcharging is not a good practice. Their decision to listen to customer demand and design the use of loose AA bats in the scanner instead of their "packs" forced them to put some of the responsibility of battery charging on the user.
I was just thinking yesterday about how well it is working for me to use the maha charger with the uniden 396xt and its loose AA battery option. THANK YOU UPMAN and UNIDEN for building your scanners with that feature - it allows me to manage my batteries to the optimum runtime which means alot with a handheld scanner! I remember back in the late 90's on the Strong Signals web forum how many of us were wishing for loose AA option and variable bank sizes. Uniden listened and now we have these wonderful Dynamic Memory Allocation scanners.
We use external smart chargers for at least 2 reasons.
1- you don't risk overcharging the batteries in the scanner, which is a significant possibility if you have a long charge timer set on the scanner. say you let the scanner charge for 16 hours and its nearly done, then you unplug it to go out and water the garden or something, then come back in and plug the scanner back in for the night, get ready for alot of overcharging, because its going to charge for 16 more hours and might only need 1. the scanner has a predefined charge current that doesn't change based on battery capacity or the battery's need to be charged or not. As upman said, capacity divided by 10 should be a safe current to overcharge a battery without melting it down and damaging your scanner, but it WILL reduce battery life over repeated exposures.
and 2- its better for the batteries to be charged slowly and to just the right level. Smart chargers like the mh-c9000 stop the charging at the right point. A timer charger in the scanner does not.
Now I understand that uniden couldn't put a charger as advanced as the maha in their scanner, and the fact that they put a settable charge time setting in the scanner is proof that they understand that overcharging is not a good practice. Their decision to listen to customer demand and design the use of loose AA bats in the scanner instead of their "packs" forced them to put some of the responsibility of battery charging on the user.
I was just thinking yesterday about how well it is working for me to use the maha charger with the uniden 396xt and its loose AA battery option. THANK YOU UPMAN and UNIDEN for building your scanners with that feature - it allows me to manage my batteries to the optimum runtime which means alot with a handheld scanner! I remember back in the late 90's on the Strong Signals web forum how many of us were wishing for loose AA option and variable bank sizes. Uniden listened and now we have these wonderful Dynamic Memory Allocation scanners.
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I will chime in and say most modern NiMH can handle a 1C charge rate with very little if any affect on lifespan. So 1600mah batteries at 240ma (externally) would be fine hell i would charge them at 1.6amp or even 3.
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SCFast: when you say "charge" I hope you don't mean hold indefinitely at those charge rates, but rather are talking about the setting on the smart charger which will remove those high rates once the battery is sensed as full.
I'd agree that you can charge fast, but if those fast charge rates are held after the battery is full, hold on to your hat. (its important to differentiate safe charge rates when bat is not full vs when the bat is full, which as it relates to the scanner's time based charge method is part of the reason for all this discussion)
I'd agree that you can charge fast, but if those fast charge rates are held after the battery is full, hold on to your hat. (its important to differentiate safe charge rates when bat is not full vs when the bat is full, which as it relates to the scanner's time based charge method is part of the reason for all this discussion)
WOW! thanks for all the great info. really appreciate it and will take heed.
-bill
-bill
Actually, to reliably cutoff charge for NiMH batteries you need to charge at a 1C rate or higher and only charge a single cell at a time (the MAHA charger you mention does this).
nd5y
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If you want to learn more about batteries check out Basic to Advanced Battery Information from Battery University
Another advantage of charging in a smart charger (I use a LaCrosse BC-900) is that if I really need to, I can charge a set of batteries up pretty quick by selecting a higher rate for charging. I don't like to if I can avoid it, but when you need it, it's there.
Everything works when new
Until they age. Then they can't handle the high charge rate, and get HOT, and if in the scanner, can melt the case.
There are no consumer products (other than actual battery chargers) that I am aware of that charge loose cells internally at anywhere near 1C (typically .1C max).
NiMH batteries, even when new, get quite warm during charge. This is because the chemical reaction involved in charging NiMH batteries is exothermic (generates heat). This is quite different from NiCd chemistry which is endothermic during charge and only gets hot during overcharge.
If you use a delta-v cutoff charger at much less than a 1C rate with NiMH batteries you risk not detecting the full-charge state and continuing to charge until timer expiration. This is true regardless of the age of the batteries. Really there is a window between around .2C and .8C you should not charge at regardless of cutoff type. >.2C is too much power to dissipate any overcharge entirely as heat with a timed charge. <.8C begins to make delta-v cutoff unreliable.
NiMH batteries, even when new, get quite warm during charge. This is because the chemical reaction involved in charging NiMH batteries is exothermic (generates heat). This is quite different from NiCd chemistry which is endothermic during charge and only gets hot during overcharge.
If you use a delta-v cutoff charger at much less than a 1C rate with NiMH batteries you risk not detecting the full-charge state and continuing to charge until timer expiration. This is true regardless of the age of the batteries. Really there is a window between around .2C and .8C you should not charge at regardless of cutoff type. >.2C is too much power to dissipate any overcharge entirely as heat with a timed charge. <.8C begins to make delta-v cutoff unreliable.
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Interesting, I found this:
Maha / Powerex Corporate Site : NiMh Batteries and Chargers from Powerex, Digital Cameras from Maha, Powerex 2200 Rechargeable Batteries and Chargers, AA, nimh, lithium ion, 2000 mah, 2200mah, charger, battery
The recommended charging current is 0.5C, or 0.5 times the battery capacity. The recommended discharging current is 0.25C, or 0.25C times the battery capacity.
So how to figure out the optimum charge? The Maha folks are recommending a lower rate, and presumably this will result in lower heat and lower life.
If we don't pick a high enough rate, then the charger can't detect end of charge and goes into 'auto timer off' mode...
Here is what Maha thinks is a too low rate:
When charging below 0.33C (except in BREAK-IN) mode, the batteries may not produce a sufficient end-of-charge signal for the charger to terminate correctly. Although the temperature sensors will safeguard battery overheating, lower charging rate might not cause enough heating in the batteries to trip the sensors.
Anyway, agree with all other points on the thread. External charger is more flexible and allows optimum use of a variety of batteries. I'm currently running Eneloop batteries and it was very useful to be able to research Sanyo web site to see how their chargers were speced for charging currents. Then I could plug those values into my Maha...
I will admit that my info is based on data from a couple of years ago. There may have been advances in either chemistry or charging technology that allow safer cutoff detection at lower charge rates. I'll go with what MAHA says. 
Better designed/programmed chargers
From my point of view, this is a HUGE firmware bug. The detection of End Of Charge should have NOTHING to do with the user settable charge rate. Chargers should momentarily set (for millisconds at most) the charge rate as needed to detect proper EOC.
That is not possible using NiMH batteries being charged in series. Change the laws of physics and we'll look at changing the firmware.
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