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Hybrid electric vehicles use 12 V for most purposes (other than traction and aircon), and they generally come with a 12 V ‘auxiliary’ battery (lead-acid, about 45 Ah) charged from the vehicle DC-DC converter. For some owners, it would make more sense to use LFP chemistry for the auxiliary battery (which does not crank an engine). The vehicle that I have tested (a 2019 rav4 hybrid) does seem to sense and send appropriate voltage to a 50 Ah LFP battery (14.2 V bulk stage, dropping to 13.6 V once the battery is charged). But at low SOC, an unregulated LFP battery can draw too much current for either the battery (1C) or the vehicle circuitry (~100 A). There is a single cable to deliver charge to and from the battery. Interposing a DC-DC charger would limit current to the battery, but these are generally unidirectional, so the battery can no longer provide power as needed. Bidirectional DC-DC converters exist, but currently they seem to be very expensive. Is it as simple as adding something like a Schottky power diode back from the battery to the vehicle circuit? Is there a better solution?circuit?

I have looked at this thread using-lifepo4-battery-as-car-battery but it does not answer my question.

RGeB
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  • Are you asking this from the point of view of adding to an existing vehicle or to design it into the vehicle? Many cars do have current sensors on the battery (in the ground lead on the ones I've seen) so can modulate the 12v bus voltage to manage the battery charging current. – Kevin White Apr 15 '21 at 23:48
  • You didn’t define a need for bidirectional charger, only for low SoC LPF which you ought to be able to current limit the charger to CC then CV and cutoff. So I don’t understand what you need except not a CV battery charger.which is essentially a series switch from alternator. The charger should not be active when not running unless you specify this then you need to monitor SoC of main battery when ignition is off. Battery voltage sensing ought to be independent of load current to prevent line loss hysteresis. If it can’t or shouldn’t power the LPF how can the main battery the car? – Tony Stewart EE75 Apr 16 '21 at 01:23
  • Simplistically you could do as shown. Better than a Schottky would be a FET with reverse to usual DS polarity switched on when battery is discharging. (That way body diode conducts during discharge even if FET is switched off.). In practice the charger should be tailored to LiFePO4 (as you probably intend) and whole system designed with LiFePO4 in mind. | Your example bulk/float charging is more like lead acid than CCCV Alkaline charging. Floating at below Vmax (here 13.6V/4 = 3.4V/cell is (IMHO) questionable for LiFePO4. – Russell McMahon Apr 16 '21 at 01:41
  • Kevin: Adding to an existing vehicle (as mentioned a 2019 rav4 hybrid). This model does have a (hall effect) current sensor on the auxiliary battery negative terminal, but Toyota does not seem to disclose how it is used. On the ICE rav4 I believe it was used to control the 'smart' alternatorb but I think there would be a lot of experimentation to use this sensor without vehicle error codes, so I am trying to find a simpler solution that leaves it alone. – RGeB Apr 17 '21 at 00:27
  • Tony: The hybrid rav4 uses a DC-DC converter rather than an alternator, but the need for bidirectional power is the same. You seem to have twigged to this at the end of your comment. The interposed DC-DC charger can provide the appropriate charge profile for LFP when the vehicle is running, but as I said, these are generally unidirectional and (as in most cars) there is a single cable to deliver charge to and from the battery. So, some way is needed to let the battery provide power when the vehicle is not running (in ready mode). How best to do that was the question. – RGeB Apr 17 '21 at 00:39
  • Russell: the existing vehicle is designed for lead-acid, but an interposed DC-DC charger can provide optimal charge profile for LFP (voltage, current and in some models even the desired current-based charge termination). I think you are saying that a Schottky diode or FET should work to allow power back into the vehicle circuit from the battery. That sounds good. Could you provide a diagram for how to wire an FET for this purpose? Would it be as robust with currents around 50 Amps into sometimes inductive loads (like solenoids that might be opened during various vehicle functions)? – RGeB Apr 17 '21 at 00:51
  • From the first-posts review queue: I recommend splitting the long paragraph into more than one. – Nike Dattani Apr 24 '21 at 19:56
  • My prius came with one for its aux battery, I've recharged it a few times, when I let it die on accident, seems to be working OK still, despite a fairly cold climate [UT]... :) – rogerdpack Mar 30 '23 at 23:23

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