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Old June 19th 18, 03:24 AM posted to rec.aviation.soaring
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Default A complicated question about LiFePO4 batteries, - and switching.

On Monday, June 18, 2018 at 9:22:24 PM UTC-4, wrote:
Another question regarding the above battery type! There are variety of techniques used when switching between batteries - on/off/on switches, good switches with a cut out between the movement, two batteries with a diode to prevent flow between both batteries, etc.
The question I have concerns an older glider with limited battery space (Libelle), previously using SLA batteries and and a simple switch between the two. I will be upgrading with a new panel, likely LiFe batteries and want a back-up as well. While I understand the function of the diodes in the switching circuit, is there any reason to be concerned about their use with the LiPo battery and the internal Battery Management System? Imagine two LiFePO4 batteries that have equal charges, only one is discharging at a time until some threshold is reached and they switch over as determined by diodes outside the internal battery system. Is there a reason to question this type of circuit with the use of this new battery technology and the Battery Management System within?
While I love these new batteries, I don't want to bring on any new problem in the air, being quite aware that the wiring is only to contain smoke.

Thanks ahead, Mark


There shouldn't be an issue, as long as both batteries have diodes to ensure that current can only come out of the batteries, not into them. The BMS will still serve as a second line of defense to limit the current (in either direction) if needed.

The only downside of the diodes is that you lose a bit of the battery voltage, since the diode has some voltage drop between its terminals. You can use Schottkey type diodes to minimize that voltage drop, about 0.3V as compared with 0.7V with a standard silicon diode. This is less of a problem with LiFe batteries, since their voltage is somewhat higher than that of SLAs, especially after they're partially discharged.

An arrangement that can be used to eliminate the voltage drop, especially if using SLAs, is to have both the two diodes and also an SPDT or on/off/on switch, from the junction of the two diodes to the other end of either diode. (Or you can think of it as the normal switch arrangement, with the diodes added in parallel with the two arms of the switch.) That way you get the full voltage of whichever battery is chosen by the switch at the time, but if you forget to switch over, or if the switch fails, you still get the other battery to kick in if the first one gets weak. E.g., with normal silicon diodes, if the first battery is down to 12.0V and the second one is sitting at 12.7V then the diode in series with the second one will start conducting, keeping the voltage supplied to your gizmos from falling lower than 12.0V at first, decreasing slowly from there as both batteries work effectively in parallel albeit at different voltage levels. If you then remember and flip the switch you'd get the full voltage of the second battery. Moreover, with this arrangement, if the switch momentarily disconnects both batteries while you flip it, there is no supply gap to your gizmos since the current can keep flowing through the diode. More precisely, the momentary dip in voltage is limited to the small voltage drop in the diode.