Adding a second battery to my cmax

[WhiteKnight]

I am new to hybrid cars and to my cmax (2015) so if this question has an obvious answer forgive me.

 

What I want to do is attach a second 12V battery to my cmax (not replace one, have two batteries). Since the hybrid doesn't have a conventional alternator I can't use a typical two battery set-up that conventional cars can use. So what I thought about doing was using a B2B charger (such as one linked to below) to charge the auxiliary battery.

 

https://www.sterling-power-usa.com/SterlingPower12volt-12volt-30ampbatterytobatterycharger.aspx

 

This would mean, however, that the puny 12V battery would now be regulating the voltage of not only the hybrid battery but the 12v auxiliary battery too. To visualize:

 

Hybrid battery<---cmax12v battery---->12v auxiliary battery.

 

Can the cmax 12V battery manage that safely without messing up the hybrid battery or the car's electronics?

[fbov]

I am new to hybrid cars and to my cmax (2015) so if this question has an obvious answer forgive me.

 

What I want to do is attach a second 12V battery to my cmax (not replace one, have two batteries).

Why? You've got 1.5KVA already, perhaps it already meets your needs. 

Frank

[WhiteKnight]

Why? You've got 1.5KVA already, perhaps it already meets your needs. 

Frank

Why? Prudence. In theory the hybrid battery meets needs in terms of capacity but probably not in fact.

 

I have several power accessories that pull between 900-1000 watts each. I only run them for short bursts of time (4-5 minutes) 1-3 times a day. That power draw is much too big for the built-in 150W plug so that would mean some type of direct connection to the hybrid battery--for a newbie like me, probably not a great idea. Also, I don't know what effect that level of power draw would have on the longevity of the hybrid battery.  Seems like a bad idea, but maybe I am wrong.

 

So that is why I want a second battery, to protect the hybrid battery from the high power draw of my accessories. If I draw to much power (say, by forgetting to turn one off) I've only killed a $200 item rather than a $3000 item.

[ptjones]

I think it would be better/safer to use the second 12v battery separately with a 1000 watt inverter and charge it from household current. :headscratch:

 

Paul

[fbov]

Yep, that needs a buffer battery, especially since you've got the rest of the system in place. Two options are a low-power trickle charger that doesn't overload the 110v outlet, or perhaps the B2B charger using the full-time 12v outlet in the cargo area. Both require the vehicle running, the 110V to operate, the 12v for maintenance. Bad things happen if the car's 12v battery gets weak/low, regardless the state of HVB charge. Be careful not to draw it down. Early SE's had issues that drove owners nuts. 

 

You can test your 12v battery with the "radio test." The radio stays on after you turn off the car, until you open the door. It eventually times out; measure how long it takes. 10 min. is very good, 2 min. not so much... unless the battery was changed and the SW not reset. Do a search here if you're interested. 

 

HAve fun,

Frank

[plus 3 golfer]

What input voltage do your power accessories require?

 

The C-Max has a DC/DC converter capable of delivering around 145 A at a 12V battery charging voltage between 13V and 14.9 V. The C-Max 12 V battery when new is rated 43 Ah and fused at 150A. So, since your accessory loads are around 1000W for 5 minutes several times a day, the converter and battery should be able to power the accessories. The HVB is all ready protected.. The 12 V battery will “buffer” the power swings of your accessories (it’s a big capacitor).

 

IMO, All you need to do is start the C-Max and turn off all C-Max accessory loads - lights, fans, radio, wipers, seat warmers and so forth. Your accessory load must be wired as follows: 1) 12 V Positive a) to the load side of the 150 A fuse (so if your load shorts + to ground the 150 A fuse blows or b) to a an appropriate sized fuse for your accessories off the + battery terminal and 2) the 12 V Negative to chassis ground not the 12V battery terminal. The negative battery cable has a metering circuit that the BMS uses to integrate the current flow in / out of the 12 V battery to estimate Its SOC and Ah capacity losses (as the battery ages) for use in the the 12 V battery charging algorithm.

 

So, I don’t believe you need a B2B or a second battery. Depending on your accessory input voltage requirements you might need a DCDC converter or a DCAC inverter. i’d spend the $ to get one with higher efficiency that meets the power input specs of your accessories. Your car engine will auto start if the HVB SOC gets too low.

Edited September 15, 2019 by Plus 3 Golfer

[62Lincoln]

I think it would be better/safer to use the second 12v battery separately with a 1000 watt inverter and charge it from household current. :headscratch:

 

Paul

x2 Agree