Please read this guide which will cover all the kinds of batteries you can use.
The device you are charging is almost certainly very 'stupid' - it doesn't know or care that you are trying to use AA batteries to recharge it. Most devices assume that you are plugged into the wall or into a computer (which is plugged into the wall) and suck as much power as possible to quickly charge up so you can listen to music or make phone calls.
This, of course, is a bit of a tradeoff. The higher the current draw, the harder the mintyboost has to work, and the AA's have to work. They'll get hot, they lose efficiency, the batteries drain and droop. There is no way for the mintyboost itself to fix this, it is completely up to the phone or music player!
For that reason, we need to have good batteries, that will be able to work well while being drained fast!
In approximate order of effectiveness, here are the non-rechargable batteries you will want to use:
- Lithium 1.5V 'high drain' AA batteries (2900 mAh)
- High quality (Duracell/Energizer) 'high drain' alkaline batteries
- High quality (Duracell/Energizer) alkaline batteries
- Non-brand Alkaline batteries
- Other kinds of non-rechargable AA batteries (zinc, etc)
Higher quality batteries will give you longer boost times, cheap or dead batteries are the leading cause of 'flaky' behavior so be sure to get good batteries.
Don't "mix" dead and non-dead cells, or different kinds of cells. If you buy 2 batteries in a pack, keep them together. This will get you the best performance.
- High-capacity (~2500mAh), high quality NiMH batteries (See the standard in testing, the NiMH shootout!)
- Non-brand medium capacity NiMH batteries (See the standard in testing, the NiMH shootout!)
- NiZn batteries (? We haven't actually tested these so we are not 100% sure if they perform well but they should be OK)
- NiCad batteries
Good quality NiMH batteries work fantastic, but be careful of using really old cells. Don't "mix" dead and non-dead cells, or different kinds/brands of cells. If you buy 2 batteries in a pack, keep them together, and charge/use them at the same time. This will get you the best performance.
The tradeoffs are:
- Alkaline batteries are available eveywhere, don't need to be charged before use, can be less expensive
- Alkaline batteries have a higher 'nominal voltage' of 1.5V so they provide more power just because of the voltage increase
- NiMH are very good at providing a lot of current, often better than Alkalines (although perhaps not as good as Lithium 1.5V cells)
- NiMH cells other than Eneloop and similar 'low self discharge' will slowly run down even when just sitting around
Because the run time completely depends on how the device charges, what its battery size is, etc, nothing beats timing and testing the first time you use the Mintyboost to charge your device.
Lets start with alkalines. Please refer to this Duracell alkaline battery capacity chart:
If you want even MORE power, you can add a third battery in series to increase the input voltage from 3 volts (2 x 1.5v) to 4.5 volts. Three AA Alkaline batteries has approximately 1/3x more power than 2 Alkaline AA's, three C's have 4x as much capacity as two AA's, and three D's have 7.5x more capacity.
,br> However, there is a limit to how many more batteries you can add. For example, after 3 Alkalines, one would think you should go for 4. But 4 Alkalines is 6V nominal (and actually may be as high as 7V) - since this is higher than the 5V output, it is not safe or good for the mintyboost or your device. For that reason, use only 2 or 3 alkaline batteries, not 4!
OK, now what about rechargable AA's? First thing to remember is that rechargables C or D cells may not have more capacity. Sometimes they are really AA batteries in a large plastic shell! Assuming that you can get real rechargable C or D cells, you should be able to get 3x or 5x more capacity just by going with bigger batteries.
The other option is to add more batteries in series. You can definately go with three NiMH cells, which will boost power by 1/3 and as long as you are using NiMH/NiCad rechargable you can also try 4 AAs. Because NiMH cells do not run at 1.5v (they run at 1.2v or so) four cells will be under 5 volts. Of course, do not use 5 or 6 AA's as that will go well over the 5 volt limit.
- Use only single lithium ion polymer cells - 3.7 to 4.2v. Don't use 7.2v-8.4v cells! those are way too big.
- Use only lithium ion/poly cell with a protection circuit. The mintyboost draws a lot of current and will drain the battery all the way down, unprotected raw cells will almost certainly be damaged, either bursting, catching on fire, leaking fluid, etc. Really, we mean it!
- Use only a proper recharger for the cells, lithium ion/poly's are delicate and cannot be charged in a NiMH charge.
- Do not parallel lithium polymer cells on your own. Power packs come with a 'balancer' circuit that must be used to keep them from discharging into each other.
Basically, don't start noodling around trying to invent your own lithium polymer cell pack!
There are few things that make a difference:
- The first thing to keep in mind is that the Mintyboost converts power from low voltage to high voltage (5V) but it is not perfect, there is charging efficiency loss. The conversion efficiency can be approximated as 80% - it may be higher or lower but it's pretty close.
- The device battery capacity is mAh. (The mWh is not useful here, because the charger inside is linear.) If your gadget has a 1000 mAh battery inside, that tells you the size of the bucket you are trying to fill.
- The mintyboost battery capacity. For fairly good batteries, this is going to be about 2800 mAh.
- The fact that the most AA batteries you get have their capacity specified for low-drain use and this is high drain use. For alkalines you can probably consider the capacity to really only be 75% of what they say it is. For NiMH, maybe 80%, and for Lithium Polymer it's 100% (they are really good at this sort of thing).
- The mintyboost battery voltage. For alkalines this is 1.5, for NiMH its going to be less, 1.2V
First, figure out what the milliWatt-hours of the mintyboost power supply is:
mintyboost mWh = Battery voltage (V) * Battery capacity (mAh) * Capacity derating
2 * 1.5V * 2850mA * 75% = 6400 mWh
This is the battery's capacity. Next we calculate the milliAmp hours the mintyboost will output at 5V:
Output mAh @ 5V = Battery mWh (mWh)/ 5 (V) * Boost Efficiency
6400 mWh / 5V * 80% = 1026 mAh output
This how much current it can provide, and for how long. Now divide this by the size of the battery it's charging. Say you are charging an iPhone 3G (which has a 1200 mAh battery inside):
# of full recharges = MB output mAh /Device Battery Capacity
1026mAh / 1200mAh = 0.85 times
So, at best, two Alkalines can get you about 85% charged up (not more than one charge!)
Lets try again with Sanyo AA NiMH 2700mAh (the best NiMH batteries there are):
MintyBoost mWh = 2 * 1.2V * 2700mAh * 80% = 5184 mWh input
Output mAh @ 5V = 5184mWh / 5 * 80% = 830 mAh output
# of iPhone 3G recharges = 830 mAh / 1200 mAh = 0.70
These batteries will give you 70% charge up. Less than Alkalines because the nominal voltage (1.2V) is less than Alkaline (1.5V)
Finally, lets do a 1200mAh lipoly battery with 3.7V nominal voltage:
MintyBoost mWh = 3.7V * 1200mAh * 100% = 4440 mWh input
Output mAh @ 5V = 4440mWh / 5 * 80% = 710 mAh output
# of iPhone 3G recharges = 830 mAh / 1200 mAh = 0.60
These batteries will give you 60% charge up on your iPhone.
Note that these calculations will give you a reasonable approximation and an upper bound. As the batteries age, their capacity decreases. The internal charger of whatever you're charging may also not be 100% efficient. Keep this in mind but don't be surprised if it's an 'upper limit.'
Another important thing to realize is that the chip inside the MintyBoost also has a minimum voltage requirement, which is 2V. So do not try to run it off of a single AA battery or similar.
Your best bet is to keep the battery input between 2V and 4.5V. Under 2V, the chip won't work, above 4.5V the chip becomes less efficiant (for extremely technical reasons that we won't go into right here) and above 5V it will not be able to keep the voltage at a steady 5V.
Try to aim for at least 2000mAh capacity of your 1.5v batteries. For Lithium ion/poly 3.7v cells, 1000mA or higher is good as well. Larger capacity batteries also have a positive side effect that they dont droop or sag as much under high loads and have lower internal resistance!