New Project. Smart Aux Battery

@twizylover:

Now I thought a lot about your Smart Aux Battery Project.
These are the results of my brainstorming:

There are only very few situations when the 12V aux battery is in action.
-just a few seconds when Twizy is powered on
-to feed the immobilizer when Twizy is powered off
-to feed the parking lights when Twizy is powered off
-to feed the hazard warning lights when Twizy is powered off
-to feed additional devices, for example OVMS, when Twizy is powered off

As soon as the Twizy is powered on, the DC/DC converter of the charger feeds all the 12V devices and the 12V aux battery is obsolet.
Only in case that there is more power consumption than the DC/DC converter can provide, the 12V aux battery is active too.
So there is not really the need to have more capacity.
As long as there is no OVMS installed in the Twizy, the standard capacity is absolute enough.
When an OVMS is installed and the charger provides trickle charging, everything is okay too.
Only in case of missing trickle charging while parking and having an OVMS installed there is the risk that the 12V aux battery becomes discharged when the Twizy is parked for a long time and/or in case off cold temperatures.
So only in case that an OVMS is installed to the Twizy, the 12V aux battery becomes discharged while the Twizy is parked.
In all other situations this means that the smart aux battery usually is charged to SOC 100% and never becomes discharged.
As far as I know, this isn´t good for a long lifetime of LiFePo cells.

Now let´s assume that the DC/DC converter of the oem charger is out of order.
In this situation your smart aux battery with more capacity in fact could be a helpful device.
But as far as I know, the red 12V warning lamp is activated when the system voltage drops below 13,6V.
Driving then is still possible, but it´s annoying when the warning lamp is active.
When your smart aux battery has a voltage of 14,6V at SOC 100%, how long will it last, till its voltage drops below the 13,6V threshold?

I don´t want to discourage you.
But at the moment I´m sceptic whether the device will make sense to many Twizy users.

I am also still not able to understand the benefit of this project completely.

The Anti-Theft feature is cool
A slightly more durable 12V battery is also a good Idea.

The rest is unfortunately far beyond my comprehension.

Für Leute mit Generation 1 Ladegerät und OVMS ist es auf jeden Fall interessant. Wie lange hält der Akku durch, wenn man den Twizy längere Zeit nicht bewegt?
Den 20 kWh-Akku lade ich oft nur einmal pro Woche.
Die Notabschaltung sollte dann den Twizy wohl länger am Leben halten.
Bevor ich meinen Bleiakku gegen LFP getauscht hatte, wurde der Bleiakku trotz Gen. 2 Ladegerät mehrmals so tief entladen, dass ich Starthilfe brauchte. Es kann wohl auch für Twizys mit eDriver-BMS oder bei denen das automatische Nachladen nicht zuverlässig funktioniert vorteilhaft sein.

TwizyChrisy schrieb: I am also still not able to understand the benefit of this project completely.

The Anti-Theft feature is cool
A slightly more durable 12V battery is also a good Idea.

The rest is unfortunately far beyond my comprehension.

To summarize everyone's key features could be;

- Anti-theft lock.
- Isolated 12/5V additional line with his own cut off to avoid excesive load on primary systems (oem 12v devices).
- Battery Canbus Status information. Integrate in real time with 3rds easy via canbus. OVMS, twizy displays... etc.
- Total switch output Off. like FIA regulation, appendix J.,or when you are going to store the vehicle for a long period... (until summer or something like that).
- More lifetime. Probably all the vehicle life, 25 years or more -4000 cycles at 80%- and many more after that.
- More capacity. With 40Ah you have x3 more than oem capacity, to enable 480Wh aprox in total.
- Less weight. On 40Ah version about 1kg below OEM. 4050g oem vs 3050g on 40Ah version. On 20Ah version about 2.5kg below OEM. 4050g oem vs 1550g on 20Ah version.
- Corrosion protection. Sealed connectors. Waterproof, Dustproof, vibration proof.
- Portable. As no need for a dedicated charger because it can charge from a 230V outlet, you can use it like a battery pack for other 12V/5V purposes when you hibernate your twizy!!
- Price. If we can reach the 150€-200€ price target, will be something that worth it.

And many other advantages from a more technical point of view, like:
- Independent but integrated system.
- Don't wear oem DC-DC line. At this point when finish our test we publish more tech info.
- Charges directly from his own 230V rectifier, dioded output.
- Open design.
- Lets main charger replacement projects easily, without thinking about 12V line. -Hopefully our next project-.
- Automotive grade electronic components on primary tasks.
- LiFePO4 quemistry. (vs agm).
- Modular solution. The case, the electronics and the cells.
- Charging discharging config schemes on BMS. So you can always wait to 20% capacity (nearly the same as total oem battery) to charge to again to 100%.

I hope this helps. Regards.

Snorre schrieb: Only in case that there is more power consumption than the DC/DC converter can provide, the 12V aux battery is active too.

Are you sure of this assumption? i don't think so. You have about 300w -360w peak with 30A on the Elips- to use with the oem DC-DC, but service battery is in parallel -we are checking that right now- So the load will be mixed always, not only when its exceeded. If you put 250W on the elips DC-DC the high temp reached will destroy the oem charger in a few hours.

Anyway, we are currently "decoding" the electrical wiring. If someone has official electrical schemes will be helpful.

Thanks in advance for your support.

When the Twizy is switched off, the voltage in the board network is approx. 13.5 V. This is the open-circuit voltage of the full charged 12V battery. When the Twizy is switched on, the OEM charger raises the voltage to approx. 14.6 V.

The current always comes from the source with the higher voltage. When the Twizy is switched on, this is the OEM charger, he delives 14.6 V
an analogy: its like water, water is always running downwards.

Snorre is right.

Berthold schrieb: When the Twizy is switched off, the voltage in the board network is approx. 13.5 V. This is the open-circuit voltage of the full charged 12V battery. When the Twizy is switched on, the OEM charger raises the voltage to approx. 14.6 V.

The current always comes from the source with the higher voltage. When the Twizy is switched on, this is the OEM charger, he delives 14.6 V
an analogy: its like water, water is always running downwards.

Snorre is right.

Ok, differentf voltages, we'll make some tests.

If you are right, supplying 14.7V from the battery will be a possible solution to not wear DC-DC line.

Thanks in advance for your suppport.

P.S.: I don't able to understand these french engineers that use a battery exclusively for ignition and warning lights. If you already have a DC-DC 14.6V line on board that you already are usintg it for everything after on too, and a very big battery that feeding it, why you create a single point of failure with a lead acid battery?. No way.

Snorre schrieb: @twizylover:

There are only very few situations when the 12V aux battery is in action.
-just a few seconds when Twizy is powered on
-to feed the immobilizer when Twizy is powered off
-to feed the parking lights when Twizy is powered off
-to feed the hazard warning lights when Twizy is powered off
-to feed additional devices, for example OVMS, when Twizy is powered off

In our first tests, i can tell you that there are more oem devices using aux battery with vehicle off, dashboard has load of the aux battery and other devices, not identified yet, with more load than the led of the dashboard an his own pcb.

Vehicle: Twizy 80 Technic 2012. 67000km. First version onboard charger.

i hope this helps.

Update 21/11/2025;

Finally, we have selected the main MCU for the BMS. The Ti hercules family, more in detail, the Ti TMS570 ics. Ti link

A bit expensive, but a really automotive ready MCU for funcional safaty. We discarded infineon and renesas because lack of documentation and acccess to info for small builders like us.

P.S.: If anyone knows a better option, we have ears open. Thanks.

IMHO not suitable at all. You need a very low power MCU, ideally with integrated Wireless. Your chosen device will need up to 1A at the core voltage Rail. How do you power this Beast with the small battery for more than a few Days?