…official word…

Well, I got the official word from Yaesu tech support. The FT-7800R draws up to 2A of current while transmitting at 5W. So, my understanding of the power/current/voltage relationship is somehow flawed. Also, the voltage range of operation is 11.7 to 15.8 VDC.

So, a 12VDC SLA battery would need a capacity of over 8Ah to give 1-2 hrs of transmit time from a topped off charge down to 11.7V.

Hmmm… however, a NiMH battery has a much flatter battery discharge curve. In fact, it looks to be the flattest. This means that it would hang out closer to it’s nominal voltage for longer, then steeply drop off at the end. For radio use, if you set up two identical radios for a battery test, one hooked to a 12V SLA, and one hooked to a 12V NiMH, the NiMH battery would give more radio use, since it wouldn’t drop below the 11.7V point until the very end, where it would just fall off to whatever the discharged voltage is for the battery, usually between 9 and 10 Volts.

This means that I could use a smaller Ah NiMH battery and get the same battery life.

Another thought is that to keep the voltage above 11.7, I could set up a battery pack with a nominal voltage higher than 12V. For example, there are 7.2VDC NiMH battery packs sold for RC cars. These typically have up to a 3.3Ah capacity. Two of those in series would give me a 14.4VDC 3.3Ah battery.

Edit: I was distracted by battery browsing and found a couple 14.4VDC battery packs. One claims a 3.6Ah capacity. It is a replacement battery for iRobot Roomba vacuums. I have one of those vacuums. In fact, I have an extra battery for one. Hmmm… I’ll need to get a special screwdriver, since the screw heads are a triangle shape, and I don’t have a special bit.

I’ve put the old battery in my Roomba and am seeing if it will charge. If it does, then I already have a battery and charger. When I want to charge it, I just toss it into the Roomba.

Of course, none of this may work. We’ll have to see. First, I need to check and see if the battery is any good. Then I have to finish my chemistry homework. Then I have to catch up on some back2dabike.wordpress.com pages and posts. Then I have to work on cleaning the garage this weekend. Then I have a bike ride on Sunday… the list never really ends, does it? 🙂


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