Request critique of BQ24450 based battery charger circuit

[Jester]

Can anyone spot any errors or ways to improve this circuit?


Battery is 12V, 10A-Hr lead acid
Load is < 0.5A

Needs to work outside in the dead of winter and in the heat of summer
Battery is supposed to be isolated if both supplies fail and the battery voltage drops to about 10.5V (perhaps I should bump it up to 11V?

The load is connected between Vbat and ground (small black triangle)

U8 http://www.ti.com/lit/ds/symlink/bq24450.pdf
U3 **broken link removed**
Q44 http://www.irf.com/product-info/datasheets/data/irlml6344pbf.pdf (Is this big enough?)

Thank you.

 

[D.A.(Tony)Stewart]

Where are your detailed specs?

why R5 in series with R6 but 1% of R6 and no tolerances.

Why only run charger at such a low threshold <11.5?

 

[KlausST]

Hi,

* the 12V comparator probably needs some hysteresis
* the other comparator ... does it ever switch?
* adding both comparator output signals with two 1M resistors.. does it work like expected?

Klaus

 

[Jester]

Update, this should be better:
All resistors are 1%, battery charged/discharged details are now on schematic.
Ambient Temp -40° to +50°
Input voltage is 15V-15.5V, @1A max., not really high enough for boost phase, but that's all that's available.
Load is 0.5A max, typical = 0.1A


When input power is available U3(bottom) is on and battery is connected and charging
When input power is not available U3(bottom) is off, U3(top) is on while battery is discharging until Vb falls below 2.62 U3(top) is then off and the battery is isolated until power comes back on.

Float voltage is 13.5V

When one comparator is on and the other off, gate voltage is 1/2 Vbat, so Q4 should be fully on VGS=1.1V

 

[BradtheRad]

Ambient Temp -40° to +50°

10.5V (perhaps I should bump it up to 11V

A fully charged battery may have its electrolyte remain liquid at such a low temperature...

As I heard it, as the battery discharges, its acid dilutes somewhat. At a sufficiently low temperature, the electrolyte can freeze. Therefore the battery can be discharged safely down to a certain point, at a given temperature.

There probably are charts that give more specific information. However in practice, it will be difficult to correlate volt reading to acid concentration. An amp-hour meter will be invaluable in this case.