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Discussion Starter · #1 ·
Hello.
I have an Opel Corsa-e EV.
It has a 12v SLA battery for the regular 12v stuff, but after the car is on, the onboard DC-DC converter supplies the 12v power from the HV battery.

I really want to swap this 12v SLA battery for a lightweight 12v Lifepo4 battery.

Since the EV doesn't need the cranking power of an SLA, it's a waste to haul around all that extra weight. Also, the self discharge of the SLA is higher so that's also wasted power (and therefore less range).

I have built several DIY 12v Lifepo4 batteries in the past, so that's not the problem.

The problem is charging: I have tried to install my 12v Lifepo4 (with Bluetooth enabled BMS), but the DC-DC converter will push a massive amount of current (sometimes 100A) into this small 25Ah battery.
It seems that the DC-DC is a constant voltage "charger" working at 14.4v.
As we know, Lifepo4 has a very small internal resistance, so it will take almost as many amps as the DC-DC will supply.
This will ruin the cells in the long run, and I'm afraid it will stress the DC-DC converter too.

So I thought about Integrating a small constant-current DC-DC step-down into the battery, using a diode to prevent charging going from the car to the battery directly (and to lower the voltage a bit).

But since the negative is common on the battery and the car, this doesn't work well.
Basically, the step-down uses the negative side to measure the amps going out to the battery. But if the negative is common, it measures 0 amps, and therefore cannot control the constant current.

I have also tried to "Isolate" the negative which then makes the step down work fine when charging, but the downside to this is that then discharging also happens through the step-down. So for example when unlocking the car, a surge of around 10-20 amps can go in reverse through the step-down, which is not wanted due to obvious reliability reasons.

Is there any way to isolate the negative side properly? Can I put a diode on the negative side?
Any other ideas on how to limit the current from. the DC-DC converter would be appreciated.

Thanks

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Can you lower the output of your DC-DC? 14.4V is 3.6V per cell, and the BMS is probably going to be chasing cells trying to bleed current to stay below 3.65 V, or it may just shut things off. 13.8V is a better target.

What about an AGM motorcycle battery? Odyssey makes some great smaller batteries. The range lost from self discharge will be measured in feet not miles ;)
 

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Discussion Starter · #4 ·
This is not a DIY EV, so I don't have any control over the DC-DC (it's a European car).
I'm looking for a solution that will not need any serious modification to the car.

Going for a smaller AGM might be a good solution for weight, but I wanted Lifepo4 for its additional advantages.

In theory, a solution would be to somehow "split" or isolate the charging and discharging functionality of the car's cables going to the battery terminals.
Is there any way to do this using diodes or something?
 

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You need a voltage difference of several volts to put a constant current function into the circuit. That means 3S instead of 4Son your battery, most likely, maybe 2S. Then boost to 12V if you have to. 😬

Seems like an awful lot of trouble to replace an AGM motorcycle/RV battery, though. "all that weight"...10lb? 😂

"Additional advantages?" Like what?
 

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The solar power community already makes what you're looking for.

It's called a DC-DC charger , they use it on boats and RV's with solar panels , it allows them to recharge their lithium batteries with the engine alternator ( which is 14.4 volts) a 20 amp system costs about $100 and is about the size of a book

Edit:
You might need a filter capacitor on the 14.4 volt side to even out the pulses from the power supply.
 

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Again...2.4 volts difference when running off an alternator to get the current control.

You stated your charger was at 12V and your batteries are at 12V. The magic floobydust is gone and the box likely won't work unless they pull some more expensive and less efficient tricks, so do your homework.
 

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Discussion Starter · #9 ·
I looked at the Victron Orion TR Smart (Non-isolated).
In theory it looks like it could help me. But my use case is a bit different than what it's intended for: I have only ONE battery (the 12v "starter" battery) and an "alternator" (the DC-DC converter onboard the EV). All I want is to limit the charging current to the battery from the EV’s DC-DC converter. Discharging should not be limited.

My problem is the connection: Since the positive and negative leads to/from the battery are used for both charge and discharge, I don't have separate "input" and "output" connections.

I'm not sure DC-DC B2B chargers will handle this correctly.
 

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Take a look at the solar charge controllers. You may be able to hook up your DC-DC to the PV input port, and CC will limit the draw to its rated power. Here is a random example
 

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Just put the all power leads on the 14.4volt terminal and place a diode between the charger terminals so when the lithium battery is a higher voltage than the power supply ( it's off ) it can power the system.

That should also eliminate the need for a filter capacitor.
 

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That won't charge said battery though...
Took me a minute to figure out what you meant.

The main "charger terminals" only hook up to the alternator(+) and the battery(+) .

It basically sits between the battery and the alternator and acts as current limiting device.
( with some voltage regulation)

It also turns off if it senses the alternator is off.


You put a diode in parallel with the charger so that when the 14.4v power supply turns off the lithium battery can back feed through it to power the system.

So when there is 14.4v available it has to go through the charger , but when the power supply turns off the battery can by-pass the charger to power the system.
 

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You clearly understand what you are talking about...how about a hand drawn diagram? I still don't understand where the magic of current regulation comes into play with your diode contraption.

All it sounds like you're doing is diode ORing two power sources, but you've blocked the current path, by definition, from the alternator to the battery to get that automatic switchover to the loads.
 

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i saw your photos in the OP and thought is was a home-built diy project.

You mean to say that all that scattered wiring and tiny DCDC board hot-glued to the cover is all OEM from Opel? It looks like a major fustercluck. That is taking cost savings to a ridiculous level.

No contactors between the Pack HV and the DCDC converter? Are there any contactors?

Is all the vehicle 12V supply coming from the small red wire on the right side of the DCDC board? Does it route to a fuse box somewhere.

So the pack negative is connected to chassis ground of the 12V too?

i agree with RM, some sort of wiring diagram is needed here.

This system is hanging together by a needle and thread, you had better forget about making mods to a different starter battery, it is likely only gonna work in the OEM cludged configuration; if you try to make mods it will break.
 

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You clearly understand what you are talking about...how about a hand drawn diagram? I still don't understand where the magic of current regulation comes into play with your diode contraption.

All it sounds like you're doing is diode ORing two power sources, but you've blocked the current path, by definition, from the alternator to the battery to get that automatic switchover to the loads.
I think he meant isolating the two voltage sources in a fashion like this :

but indeed there is still the problem of charging that battery at a particular current... Btw, how much voltage drop diodes will add ? 0.5v or so ?
 

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@cricketo, how does the dcdc charge the battery in your falstad diagram? The wiring does not appear complete
That's exactly what we were talking about earlier - there is no path for the battery to charge as presented. You could solve that by having a little DC-DC converter in there - many will have a voltage sag when their current is maxed out, thus they'll self-regulate. But a solar charge controller will do all of that out of the box.
 

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You guys are being thrown off by the word "Charger" being misused by the manufacturer.

It's not actually a "charger" its just a current limiting device that's in SERIES with the battery and you can program it for different battery chemistry's.

Note if the 14.4 power supply and battery are "ungrounded" you'll have to run a lead from batt(-) to the "charger" and keep it insulated from the frame.

You put in a diode , rated for load values , across the terminals so the battery can by-pass it when the power supply voltage is lower than the battery.

It's really simple and you guys are over-thinking it.



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