LiPo cell switching circuit

Hi

I have a 5 cell Lipo pack which is used on my bicycle to power the head light. The pack has balance charger taps. On longer bike trips my GPS needs an alternate power source to top up the battery. My thought was to use the 3.7V of one of the taps to power/recharge the GPS. The problem is this would seriously unbalance the pack since the Gps draws about 125ma.

So does anybody in here know of a switching circuit ( solid state or relay) which would switch between the taps to spread the load in rotation , say every few minutes so over the course of a few hours use the pack would remain more or less balanced?

I was originally thinking to use a series of small relays, and a timer like a 555 to switch through the cells, but small size and low power drain would lead me to go solid state. Any other suggestions.

Thanks for any help .

My Garmin aircraft GPS495 will run on any voltage between 7.5V and 35V; it has an internal switching power supply. Since it is a switcher, the current at 35V is about 1/4 of what it would be at 7V, so the efficiency is always high..Check if yours has a wide input voltage tolerance.

I think it is a really bad idea to "tap" any battery bank of like cells. Even if your GPS won't tolerate the higher voltage, why not use an efficient buck down-converter to run it off the 5 cell voltage. I have several old automotive cell phone chargers that use a switcher to convert 12V to 5V @ ~300mA. They are about 85% efficient, and could easily be hacked to get 3.7V instead of 5V.

My Garmin Edge 205 is has an internal litium battery of 3.7V, and charges off of a USB 5V. I looked into the Dimension Engineering DC to DC converters but at that high of input 18.5V to that low output of 5V in the current range I need the efficiency would only be 65% maybe 70%.

I do have a balance charger which is what I use to charge this pack, so any cell differences caused by tapping the cells should be adjusted on recharging, right?

Consider a battery eliminator circuit (BEC). They are sold for model airplanes and cars so the radios (4.8 to 6V) can be powered by the same battery pack as the motor. Both linear regulator and switching regulators are sold. What is appropriate for you depends on the power needs of your Garmin. Castle Creations (maybe just Castle, now) is a very popular brand in the USA.

John