Thursday, March 10, 2016

Using AA size NiMH batteries in the Yaesu FT-817ND

The FT-817ND is normally supplied without a rechargeable battery pack, but with an 8 cell AA battery tray. This is a similar arrangement to the old FT290R which had an 8 cell C size battery tray, although the FT290R allowed for fitting and charging of NiCAD cells in situ, something that can’t be done safely within the FT-817 (see safety issues below).

Alkaline cells are not suitable for use in the radio because they can not cope with high current loads and will run down more quickly than their capacity would suggest. NiMH cells are much better at handling the load so Yaesu manufacture a special pack that replaces the cell tray. This is quite expensive so I have been looking at utilising existing AA cells which I have a good stock of. These are Vapex low self discharge rate NiMH cells rated at 2500ma/hr.

Eight of these particular NiMH cells give a total voltage of 12V, but this drops to 9.6V very soon after they start to discharge. The FT-817 will operate at low voltages and is actually at its most efficient on receive below 10V (see http://www.ka7oei.com/ft817_pwr_opt.html). On transmit, with internal batteries, the power is restricted to 2.5W. Again, there is some efficiency saving at lower voltages (see http://www.k6xx.com/ft817/txcurrnt.pdf).

Calculating battery life

Our pack of eight NiMH batteries will deliver 9.6V at 2500ma/hr. Assuming we were able to utilise 80% of this capacity before the voltage dropped below the radio’s workable voltage this would give us a real life capacity of 2000 ma/hr

Current drain at 9.6V:
TX: 1400 ma @ 2.5W (on 5W this would be 1900 ma, but the radio restricts itself when running from internal batteries)
RX: 300 ma

Current drain at different transmit/receive ratios:
1:5 ratio = 520ma/hr
1:3 ratio = 662ma/hr

This equates to 3.8 hours at a 5:1 transmit/receive ratio, or 3 hours at a 3:1 ratio. On receive only you should achieve 6.6 hours. This means we can assume that two to three hours of portable operation is possible using internal AA NiMH batteries in the supplied battery tray. Because these batteries are very light we could carry a second set and double this time. Carrying one spare set would allow operation for five or six hours.


Safety issues

There may be a risk of short circuiting to the battery door as the outer of the battery is metal and connected to the negative terminal. For this reason a plastic sheet will need to be fitted between the batteries and the battery door.

It is not possible to charge these batteries safely inside the radio as the charging circuitry is not correct and there is no charge protection circuit. I have a Vapex 8 position AA smart charger  that can charge a set in a maximum of eight hours (usually faster as the cells are never fully discharged).

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