Happy Thanksgiving!
I took off work super early, 3:15, so I would have more time in my lab and wanted to finish my battery for my PRC-70.
When I got my radio it came without a battery pack so I searched for one for a while, but without success until Jim Karlow offered to sell me a partial one which was all he had. That was the best I could do in two months so I went for it. It was the case itself, the outside latches, and the connectors for radio and external power. No other hardware, no brackets, nothing inside. On the plus side it was in excellent mechanical condition.
The positioning of the connector for the radio was a tricky thing to determine as the location would have to be calculated since it couldn’t be directly measured and there were gaskets which got compressed etc.. So the project was on the back burner. At one point a friend loaned me his rig with battery pack and this allowed me to measure exactly the connector position they intended. Here is a picture of the loaned pack and the one I am working on:
Note that my pack is the shorter one. His pack is somewhat taller and was meant for some kind of wet cells and was converted to D cell nicads. I made the connector bracket from a 0.125″ thick aluminum angle bracket. The connector is mounted with shoulder screws to allow some movement to compensate for misalignment. The hole for it is also oversized to allow for the same movement range as the original. You can also note the threaded brass inserts in the case.
That’s the cover plate laying there. I determined screw locations by placing pieces of lead from a pencil on all the standoffs like this and placed the blank on top and wiggled it around.
Here it is with the fuseholder installed as well as the charging connector:
You can also see the stack of 26650 cells I glued together in the upper left. They are old A123 cells, and after all these years were still charged at the standard 50%. I used a 8S2P configuration which yields a fully charged voltage of 26.4 VDC. You may note that I didn’t use a balancer. That is because:
1) I haven’t found one I like,
2) I haven’t made my own yet (maybe this winter I’ll create a 12/24 volt one I like),
3) It really isn’t necessary if the cells start out more or less balanced, at least for a couple of dozen cycles.
It could take years for me to need that functionality and without it I don’t have to worry about it draining the cells down. I could literally leave this for 5 years and it would still be charged and ready to go.
To charge, just hook up to a bench supply at 28.8 Volts and wait until the current drops to under 50 mA.
For safety, I wired the cells together with stranded 24 AWG teflon wire. Why so thin? Extra safety in case of a short. Interestingly, I accidentally did short the pack when soldering the wires on the cells and two of those links instantly opened. It mystified me at first because there was no voltage yet everything looked fine. What had happened was that the wire opened in the middle and the teflon didn’t show it at all. Pulling one end and having it come out was a giveaway. Is this too thin? No, due to the short lengths the resistance drop is insignificant and the wires are very capable of carrying the normal operating currents.
Here it is with the stack wired and placed in. I used pretty dense closed cell foam to securely hold the block in place. I don’t know what it is, it is some special acid resistant expensive specialty stuff from work.
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And here is it with the cover placed on. I think the inserts at the end were slightly lower than the others based on how the cover is lower there but it’s not worth fixing, really. This thing feels really solid.
And here it is on the radio:
And it works perfectly. Plugged in the handset and antenna, set frequency, tuned, and talked.
So, I wondered, it’s a pretty heavy radio, how do all these sets compare? So here are the weights of my and other pack sets:
PRC-70 with tall nicad pack: 29.5 lbs
PRC-70 with short lithium ion pack: 24.0 lbs
RU-20 with lithium ion pack: 16.0 lbs
Syncal 2k with lithium ion pack: 12.5 lbs
If I had to carry a set into the field on my back I’d obviously choose the Syncal 2000. Actually, I could carry two of those and it would weigh about what the PRC-70 does!