DIY "Mild" Plug-in Prius Project: Grid-powering the 12v side w/ deep cycle batteries

[unixrocks]

Originally Posted by Daox

All chargers charge 12V batteries at ~14V. Your regular alternators put out this voltage and thats what the car runs on since its constantly running.

Ok - so why not use a 14V AGM battery like the D1400 from XSpower and charge that at night with a onboard charger?

Even then - would you need to shut off the DC-DC converter to see gains or would the DC-DC converter be smart enough to not charge the battery until it drops below 11 volts?

[Daox]

Interesting battery. I've actually never seen a 14V battery. I'd also have no idea where to get a 14V battery charger.

Link (its at the bottom):
http://4xspower.com/products/agm/

[unixrocks]

I think you have to build one..or use a 14V wall wart to trickle charge it.

This (DIY charger link below) seems better because it would be easy to build and its pretty flexible - so if you decide to build it and use it for a bench charger you can set it to whatever size battery you need to charge. Just need to replace the 12V transformer with a 16V which should be easy to source.

This also is a very small charger so it would easy fit into a case that can be mounted near the battery for onboard charging.


http://www.aaroncake.net/circuits/charger2.asp?

[unixrocks]

I gotta say - the hillbilly battery - pure genius!!!

[unixrocks]

Ok taking another look at the DIY charger I think the only thing needed to make this viable as a onboard charger is the addition of a relay between the + terminal of the battery and the final leg of the charger to keep the charger from sucking power from the battery when its not on. A diode wont do because the circuit wouldnt be able to tell what the current voltage is on the battery but relays are pretty cheap anyways.

Other than the relay addition and the upgrade to a 16V Transformer I think the circuit would work fine.

If my thinking is right the resting voltage on this battery is 14.7-14.9V. So switching off the converter the battery should be able to handle the electronics just fine for short trips. (50AH means 300watt load would give it about 2.5 hours of run time)

When the battery gets low (too bad the energy screen dont show the system voltage - would have to rig a voltage meter but I think I saw one that plugs into the cigarette lighter somewhere) switching on the converter would essentially trickle charge the battery and provide power to the system until you can plug in again.

Now....anyone know where that converter is and which wire to put a switch inline on to turn the thing off and on?

And in re-reading the previous posts it seems like this isnt even neccesary since the converter wont kick in until the voltage on the battery gets low which as was pointed out in an earlier post this is a mild discharge (13.8 is about 35% discharge which is fine for this battery) so in theory all you should have to do is put this battery in, charge at night and see instant gains and just when its going into the "time to charge it zone" the converter will kick in and trickle charge it at 13.8V while running the electronics.....in theory anyways. Was I reading that right about the converter not kicking on until it sees 13.8V?

Another bonus is this battery is AGM so it wont vent - especially not at the low voltages the converter throws at it. The only drawback is the battery is a bit wide so not sure how great it would fit in the stock battery holder.

[bwilson4web]

A 14 V battery is somewhat rare but 12 V batteries are fairly common. So I would probably approach the problem with a charger-inverter between the battery and 12 V. bus. First, let's look at the expected load:

500 W / 14 V ~= 35 A. output (*)
500 W / 12 V ~= 41 A. draw from battery (rough!)

So it makes sense to go with 24 V storage and draw ~20 A for battery life. Better still, 48 V and ~10 A. But the ~35 A. output remains a challenge.

* - recent data suggests 300 W is adequate

So to design the system, I would want four, 12 V. batteries with 20 Ahr capacity to keep the expected service life reasonable. The circuit would parallel charge when not under load to keep the modules as balanced as possible. That or a well balanced charger to make sure the ending voltage is the same.

When under load, the charging circuits are isolated and the batteries put in series to a buck switcher. This should deliver 92-95% of the energy to the car and 'off load' the 12 V. source.

This is not a trivial charger/inverter to design but it isn't rocket science either. The application notes from any switching power IC are more than adequate with selection of some power MOSFETs to handle the high currents efficiently. Add a microcontroller and the problem is solved.

BTW, adding a couple of 45 W. solar cell arrays would allow daytime recharging. Think of this as an Engineer kit for the 12 V. system and one that is nearly universal. It works for any 12 V vehicle, hybrid or not.

Bob Wilson

[unixrocks]

Paid closer attention this time to the voltages - the converter does indeed keep the voltages above 13V and up to (at least so far) 14.8V - it appears to depend on the SOC of the 12V battery but even a fully charged battery is going to always get a float charge from the converter.

With each state change (like the brakes being applied etc) usually causes a slight correction in voltages (goes from 13.0 to 13.1 back to 13.0 back to 13.2 etc...)

Could explain why a battery with a bad cell can really mess up your mileage. The Prius would always be dumping 14.8V into the system thougth your battery would still start the electronics fine - which means a lot of your traction battery would get drained frequently causing the ICE to come on more often.

again - just a theory that seems to fit the facts.

[bwilson4web]

Originally Posted by unixrocks

Paid closer attention this time to the voltages - the converter does indeed keep the voltages above 13V and up to (at least so far) 14.8V - . . .

Two quick questions:

• What model Prius?
• How are you measuring the 12 V system voltage?

Thanks,
Bob Wilson

[unixrocks]

Hi Bob,
It is a base model 2008 Prius (No bluetooth/navigation etc)

I am using the built-in diagnostics screen that shows the system voltage before I start driving and just watch it as drive, go througth different exercises etc...

Which brings up another exercise...I gotta measure the voltage from the cigarette lighter/lights etc...to see if any systems are regulated.

[unixrocks]

I found the plug-in to your cigarette lighter voltage meter and picked it up today.

The power receptacles are not regulated to 12V.

I wondered about the accuracy of it but it appears to be dead on - in fact....this meter measures volts to two decimal places and the voltage sensor on the Prius displays it like 13.8 volts and what I found is the Prius is rounding up and down on the voltage.

For instance 13.44 volts on the meter shows 13.3 volts on the prius when it inches up on the meter to 13.45 volts the display on the prius changes to 13.4 volts.

I found that both to be a validation of the meters accuracy and a interesting but likely useless bit of info that the sensor works like that on the prius.

The big take away thougth is at least the power receptacles are not voltage regulated (perhaps they are once you get to 15V). Need to check the other systems (lights etc) maybe this weekend I will dig out the meter and start checking.

Also would really like to find that 14Volt converter - anyone know where that is?