DC-DC Chargers - Isolation and Sizing

[grizzzman]

I'm probably too far outside of the box here, but a couple of alkaline batteries in series with the voltage sense lead would raise that 8 volt threshold to 11.2V. Three would take it to 12.8V. If your system runs above 13.0 V except at idle, three 1.61 volt alkaline cells would do the job. And likely more accurately than a VSR. Sense current is surely low so the life of these sells should be many years (given the 10 year shelf life of modern alkaline).

That will not work. It senses the voltage through the main (current wires) Alkaline's would go BOOM! (well on a Renolgy unit)

 

[hclarkx]

That's unfortunate. And I guess I'm not dying.

 

[5teve]

I'm running 2x Enerdrive DC2DC 50amp chargers (I have twin engines 105a Alternators - one DC2DC per engine paralleled at the battery ) the charge current is adjustable so currently running 80a total.

I was told by enerdrive to add a relay on the feed to the DC2DC chargers from the crank battery controlled by the ignition. The main reason for this is that I float charge the cranks when on shore power and this raises the voltage above the activation voltage.. therefore pulling the cranks down etc. I use a 200a automotive relay on each battery - Not the simplest method.. but works for me. I dont worry about the idle current.. as the cranks should be able to keep up with requirements of the DC2DC for a bit.. and then will charge anyway. Mind you the alternators seem to keep up at our general "idle speed' as the engines are running at about 1000rpm when we are in low speed areas.

If you dont float the crank when on shore power, the DC2DC should cut off as soon as the voltage drops (the enerdrive is set to activate at 13.2v)

Steve

 

[eXodus]

Seeing how im considering the same setup an overlooked solution comes to mind. The Renogy DC to DC Charger has a 50% switch. Why not just wire a switch to that and keep it in the 50% function while the vehicle is idling. I myself have wondered if there is a way to automate such a switch based on engine idle speed. But automating such a switch is beyond my knowledge level currently. A manual switch will do so essentially you would have a 20 amp DC to DC charger at idle while the switch was flipped. When your headed down the road turn it off and charge at 40 amps.

that is good idea - I don't like DC to DC chargers in a 12V application they are inefficient compare with Relays. And have the potential of draining the starter battery. Further they are limiting charge - The highest which are out there a like 40A. I got a 180A alternator my truck doesn't need more then 50A - I want to charge the lithium bank with 100+A

So the alternator only reaches the maximum capacity when you going above I think 1800RPM.
A detection device which engages the relay (or DC -DC) as soon as you reach cruising speed?

Just had another idea - Cruise control - I'm using it always - just take the signal from that - as long as that is activated - charge and soon as it deactivates - remove charge.

 

[5teve]

that is good idea - I don't like DC to DC chargers in a 12V application they are inefficient compare with Relays. And have the potential of draining the starter battery. Further they are limiting charge - The highest which are out there a like 40A. I got a 180A alternator my truck doesn't need more then 50A - I want to charge the lithium bank with 100+A

As I posted earlier I can do 100a.. via DC2DC... mine are 50a each and can be ran in parallel.

Re charging direct from the alternator.. using LIFEpo4 batteries with large BMS's you are likely to kill your alternator with the draw the battery will put on the alternator. Alternators unless specifically designed are not designed to run flat out at full current... so your 180, may be good for 80-90a continuous... unless its specifically designed for 100% duty..

The other thing the DC2DC gives you is charge profiles, buck and boost.. so it gets the right voltage every time and even with LA batteries will give a faster charge..

Steve

 

[eXodus]

LIFEpo4 batteries with large BMS's you are likely to kill your alternator with the draw the battery will put on the alternator.

I always hear that story but the only source I could find are guys which are selling DC2DC charge controllers.
Never found anything scientific or non seller sources showing this true as a fact.
I know alternators tend to burn up when large loads are put on them on idle - because the cooling is insufficient. Usually not a problem at speed.

Most tests have been done by directly connecting a lithium battery with like a foot of cable to a alternator alone - which never happens in reality Usually the LFP bank is far far away from the alternator at least a few meters and the negative is the poor conducting frame of the vehicle. So you got some resistance in the circuit which those bench tests don't have. Even one ohm makes huge difference. Further in a real vehicle you parallel the LFP to a lead battery - the lead also supplies amps to the LFP - when connecting - it's not all coming from the alternator.
You almost never charge from a fully deplete lithium battery - where the highest draw occures.

I'm happy to be proven wrong.

The other thing the DC2DC gives you is charge profiles, buck and boost.. so it gets the right voltage every time and even with LA batteries will give a faster charge..

that is a good point. Yet I'm not convinced that the downsides (aside of cost) outweigh that.

- I'm float my engine battery from the solar system, so it's constantly above 13.2V, The the DC2DC needs be controller by ignition.
- More parts to fail.
- limited charging compared when I would drop in a 250A or 320A alternator - those things are cheap ($150-200) and bolt in within like 10 minutes.

 

[peatop]

I watched a video about the affects on alternators from charging LFP batteries at high amps and one of the issues was that when the BMS in the LFP shut the battery charge of the alternator couldn't cope with the massive amount of current and burnout occurred, this was actually on a boat, also you have to remember that LFP have a very low resistance so potentially will pull whatever amps they need without being regulated.
unfortunately I don't remember the names of the videos or I would attach links.

 

[John Frum]

I watched a video about the affects on alternators from charging LFP batteries at high amps and one of the issues was that when the BMS in the LFP shut the battery charge of the alternator couldn't cope with the massive amount of current and burnout occurred, this was actually on a boat, also you have to remember that LFP have a very low resistance so potentially will pull whatever amps they need without being regulated.
unfortunately I don't remember the names of the videos or I would attach links.

The BMS should not be terminating the charge except in an emergency.

 

[Short_Shot]

Most dc-dc chargers turn themselves on and off depending on the voltage of the starter battery.

Believe it or not, most don't do that.

Only a few do, like victrons.

Edit: ancient thread.