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Old 10-31-2019, 09:04 PM   #11 (permalink)
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All of those conversions can't be efficient. :/

Guess it's not that important if you're charging it with grid power.

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Old 11-01-2019, 09:56 AM   #12 (permalink)
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My guess is each conversion is probably about 80% efficient. If you guys have suggestions I'm all ears. This just seemed like a easy and inexpensive way to get done what I am attempting. I'm no electronics expert.

That is what is great about Redpoints idea, it's very simple and has less components. The only issue I see with it is that it uses a very small percentage of the capacity of the lithium battery, and the battery is the largest cost. This is my attempt to reduce the cost.
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Old 11-01-2019, 10:01 AM   #13 (permalink)
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I see adding a lithium in parallel as doing nothing but good. While it's plugged in the 12v lead acid will cause a constant small drain on it, turning some number of watts into heat. If you try this I'd be highly interested in what kind of current you see coming out of the lithium to float the lead acid battery at ~14.5v.
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Old 11-01-2019, 12:44 PM   #14 (permalink)
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I disconnected the Pb battery when I attached the Li-ion battery because I could hear electrolyte decomposing from the higher voltage.
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Old 11-01-2019, 02:00 PM   #15 (permalink)
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Yeah, the lead acid battery can not handle a 16.8V LiPo cell connected in parallel. We could eliminate the lead acid battery as Redpoint did in his test. That makes it nice and simple, swap out the lead acid for a LiPo.

There are some great benefits to be had with going this route. First off its really simple. You shouldn't need a BMS. The alternator will not overcharge these cells. Also, the charging efficiency of the LiPos is far superior to the lead acid battery being charged near max SOC. Lead acid batteries are said to only be ~50% effective near full charge.

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These tests indicate that from about 10% SOC to 84% SOC the average overall battery charging efficiency is over 90%. On the other hand, the battery charging efficiency from 79% to 84% is only 55%. At 90% of charge and above, the charging was less than 50% efficient.
On the other hand, LiPos are said to be 99% efficient! This is a huge efficiency boost once the alternator is in use. Of course we also have the weight benefit of going from a 40-50 lb battery to half to a quarter of that.

And, there is also one big downside to this method as well, and that is usable battery capacity. I'm not too crazy about running my car's electronics at much over 15V. I know lightbulb life is negatively effected at higher voltages. I'm not sure what else is though.


Lets look at an example. Lets say we charge the pack up to 15.5V, and the alternator will start charging once the voltage gets below 14.5V. I grabbed this discharge chart from the Sonata 5300 cells that I wanted to use for this project. The two red lines represent a 4S 15.5V charge, and 14.5V. The colored verticle lines show us our usable capacity at their respective amperage draw. This shows us that the usable capacity of these 5.3Ah cells is only about 1.2-1.3Ah. That means we'll actually need a very large pack to get any reasonable usable capacity out of it.



I think the ultimate solution is basically what mpg_numbers_buy said. You have a very small 4S LiPo pack made of power cells (cells designed to put out a lot of power) that can start your car easily. It could be quite small and light. Then, you have another pack made of energy cells (cells designed for lower power output, but usually higher cycle life) that can charge the 4S LiPo pack through a dc-dc converter. This pack would be grid charged. It would be sized as large as you need to give you the required alternator off time. Once this pack runs out, the alternator kicks in. However, (of course there is a downside) it has the complication of BOTH systems.
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Old 11-01-2019, 02:49 PM   #16 (permalink)
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I think the ultimate solution is basically what mpg_numbers_buy said. You have a very small 4S LiPo pack made of power cells (cells designed to put out a lot of power) that can start your car easily. It could be quite small and light. Then, you have another pack made of energy cells (cells designed for lower power output, but usually higher cycle life) that can charge the 4S LiPo pack through a dc-dc converter. This pack would be grid charged. It would be sized as large as you need to give you the required alternator off time. Once this pack runs out, the alternator kicks in. However, (of course there is a downside) it has the complication of BOTH systems.
If you do the balancing yourself, it really isn't all that complicated.

With a lithium battery, especially a homemade one, you will want to relocate your 12V to the cabin. Leave the DC-DC connected to the 12V, and make connections going to the larger battery that you can easily remove.

The complication would involve installing it neatly, or if you wanted to engage regenerative braking.
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Old 11-01-2019, 03:02 PM   #17 (permalink)
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Excellent points! I agree.

There is still the problem for most of us and charging lithium below freezing temperatures. This is why I am going to keep the lead acid 12V, and do a lithium jumper pack. The lead acid car stay in the car and handle the cold and the lithium can be brought in to stay warm.
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Old 11-01-2019, 03:03 PM   #18 (permalink)
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Interesting suggestion... if you're providing the boost in voltage via buck converter, you could wire the brake switch to disconnect it so the alternator sees battery voltage and turns back on. You'd get regen back into the battery whenever you tap the brakes. The only downside is the alternator would stay on while you're stopped with your foot on the brake.

If instead you could signal the buck converter to disable whenever the car goes open loop, that would be even better. It would go open loop when DFCO and charge the battery, and then re-enable the buck converter once it goes closed loop at a stop.
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Old 11-01-2019, 04:33 PM   #19 (permalink)
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Hi,
It may sound primitive but how about adding a secondary battery just for the high energy consuming loads like headlights, cabin fan et cetera?and then charge it at home?it will need some extra wiring and a battery.
Has anybody done that?
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Old 11-01-2019, 04:42 PM   #20 (permalink)
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You've just suggested what this whole discussion is about.

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