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DC-DC Chargers - Isolation and Sizing

Marinepower

Solar Enthusiast
Joined
Apr 24, 2020
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I'm trying to design a system to charge a 12v -280ah LFP bank from a high output alternator as well as charge and isolate an engine start battery.

DC-DC charger seems a good way to go but there are 2 issues I'm trying to understand before committing to this route.

First, do I need to size the DC-DC charger to be less than the out put of my alternator at idle RPM?

For example in this video
(see 5 min and 45 seconds in) I see that the poster's starter battery is being drained by the DC-DC charger because his alternator cannot keep up to the demands of the DC-DC charger at idle?

I would ideally like to size my DC-DC charger to take advantage of all the current my alternator can put out. But if i'm limited to the idle output, then I may have to look for another solution.

Are others using starter battery isolation (like a Victron Argofet https://www.victronenergy.com/battery-isolators-and-combiners/argo-fet-battery-isolators ) in addition to a DC-DC charger to avoid this issue? Or is this an issue specific to the Renology DC-DC chargers and other brands have input voltage sensing protection or some other built in isolation to protect the starter battery?

Sorry if this question has been asked and answered before. I did search this forum without luck.

MP
 
Most dc-dc chargers turn themselves on and off depending on the voltage of the starter battery.
Is the starter battery sensed with the IGN lead?
I cannot find any references to this functionality, does it have a specific term i can search? Or can you point me to a DC-DC charge with this?
 
Comments on the video and possible draining of the vehicle battery at idle. Most vehicles produced in recent years have alternators rated at 150 amps and higher. Limited testing I have carried out indicate around 80 to 100 amps 'spare' at idle with vehicles fitted with 180 amp alternators.
When extra loads are placed on the vehicle system, lights, heating, the 'spare' is reduced somewhat. Most manufactures of mid sized vans , Promaster, Sprinter , Transit, recommend in their up fitters directives to limit the 'camper' loads to around 50 amps.

The video indicated a 40A charge from the Renogy unit , this implies there was around a 50A or higher load on the engine battery as the engine dropped to idle speed. Clearly with this particular van and its alternator and engine battery state and age, it would be inadvisable to continue with the setup until modifications were put in place to rectify the issue. If the Renogy unit was operating as its supposed to, then it should have shut down as the engine went to idle and the volts dropped. A number of issues have been reported with this unit in real applications rather than bench tests. This could be down to design issues or poor installation.

From your user name I guess its a marine application where its fairly easy to change alternators to higher output versions if required. Running engines at idle speed for long periods can cause mechanical issues and should be avoided.

You need to size any DC to DC charger to less than the idle output as under these conditions the alternator will be running flat out and overheating. Most alternators will deliver at least 50% of rating at engine idle revs, ( this may vary on a marine engine depending on belt pulley ratio).

As policy I dont recommend DC to DC units other than Sterling, Victron, Voltronics, all are user programmable and have voltage sensing for charge control. I think its important to be able to configure the charge voltages and profile for DIY battery packs, many of the so called pre set lithium settings seem too aggressive.

Mike
 
I'm trying to design a system to charge a 12v -280ah LFP bank from a high output alternator as well as charge and isolate an engine start battery.

DC-DC charger seems a good way to go but there are 2 issues I'm trying to understand before committing to this route.

First, do I need to size the DC-DC charger to be less than the out put of my alternator at idle RPM?

For example in this video
(see 5 min and 45 seconds in) I see that the poster's starter battery is being drained by the DC-DC charger because his alternator cannot keep up to the demands of the DC-DC charger at idle?

I would ideally like to size my DC-DC charger to take advantage of all the current my alternator can put out. But if i'm limited to the idle output, then I may have to look for another solution.

Are others using starter battery isolation (like a Victron Argofet https://www.victronenergy.com/battery-isolators-and-combiners/argo-fet-battery-isolators ) in addition to a DC-DC charger to avoid this issue? Or is this an issue specific to the Renology DC-DC chargers and other brands have input voltage sensing protection or some other built in isolation to protect the starter battery?

Sorry if this question has been asked and answered before. I did search this forum without luck.

MP
I have the 40 amp Renolgy DC to DC charger. I have a 220 amp Alt. So it keeps up just fine at idle. the unit is powered up by a 12 volt signal from the ignition switch. Auto disconnect will not happen tell 8 volts. It has the ability to reduce the amps to 12.5 percent. I would not worry about short idle stops. Long idle times could use the reduction feature. It is a buck/boost device. (It works well) I suggest that you spend time reading the manual.
Renolgy DC to DC Charger Manual
 
I have a Sterling, it works great, the setup is a bit esoteric but you only have to do it once. I would stay away from Renogy products, that's based on my feelings more than anything, there always seems to be one problem or another that people are running into with their products or customer service.
 
I have a Sterling, it works great, the setup is a bit esoteric but you only have to do it once. I would stay away from Renogy products, that's based on my feelings more than anything, there always seems to be one problem or another that people are running into with their products or customer service.
The 25V max solar input voltage of the Renogy DDC50 makes me wonder what they were thinking.
 
Comments on the video and possible draining of the vehicle battery at idle. Most vehicles produced in recent years have alternators rated at 150 amps and higher. Limited testing I have carried out indicate around 80 to 100 amps 'spare' at idle with vehicles fitted with 180 amp alternators.
When extra loads are placed on the vehicle system, lights, heating, the 'spare' is reduced somewhat. Most manufactures of mid sized vans , Promaster, Sprinter , Transit, recommend in their up fitters directives to limit the 'camper' loads to around 50 amps.

The video indicated a 40A charge from the Renogy unit , this implies there was around a 50A or higher load on the engine battery as the engine dropped to idle speed. Clearly with this particular van and its alternator and engine battery state and age, it would be inadvisable to continue with the setup until modifications were put in place to rectify the issue. If the Renogy unit was operating as its supposed to, then it should have shut down as the engine went to idle and the volts dropped. A number of issues have been reported with this unit in real applications rather than bench tests. This could be down to design issues or poor installation.

From your user name I guess its a marine application where its fairly easy to change alternators to higher output versions if required. Running engines at idle speed for long periods can cause mechanical issues and should be avoided.

You need to size any DC to DC charger to less than the idle output as under these conditions the alternator will be running flat out and overheating. Most alternators will deliver at least 50% of rating at engine idle revs, ( this may vary on a marine engine depending on belt pulley ratio).

As policy I dont recommend DC to DC units other than Sterling, Victron, Voltronics, all are user programmable and have voltage sensing for charge control. I think its important to be able to configure the charge voltages and profile for DIY battery packs, many of the so called pre set lithium settings seem too aggressive.

Mike
Mike,

thanks for you post.

Yes this is for a marine application. My alternator puts out 40amps at idle and 80amp at cruising. This is a sail boat so the engine will only run an hour a day while getting to and from anchorage. Hence the need to maximize charge current during this short window.

I was planing on allowing my BMS (Electrodacus SBMS ) to handle to the LFP charge cut off, by cutting power to the ignition sense wire on the DC-DC charger.

BTW - as marginal a DC-DC charger as the Renology is, this model actually has a DIY - LFP friendly charge profile option of 14v with no absorb float or temp compensation.

The Sterlings do not seem to have great charging profiles for LFP. The Victron's look good and feature rich they are very expensive and I would need at least two. The chargers will be as much $ as the battery bank.

I think DC-DC chargers work great for an RV where you are driving a long distance but maybe not ideal when you want to maximize your alternator charge in a short period of time.
 
I have the 40 amp Renolgy DC to DC charger. I have a 220 amp Alt. So it keeps up just fine at idle. the unit is powered up by a 12 volt signal from the ignition switch. Auto disconnect will not happen tell 8 volts. It has the ability to reduce the amps to 12.5 percent. I would not worry about short idle stops. Long idle times could use the reduction feature. It is a buck/boost device. (It works well) I suggest that you spend time reading the manual.
Renolgy DC to DC Charger Manual
Thanks Grizzman. I did read the manual but missed the part about the 8 volt auto disconnect feature. 8 volts is very low, so perhaps not ideal. If I could figure our how to automate the amp reduction feature that also might be a work around. I am not knocking the Renology. Two of 40 amp chargers could be ideal in my application.
 
Thanks Grizzman. I did read the manual but missed the part about the 8 volt auto disconnect feature. 8 volts is very low, so perhaps not ideal. If I could figure our how to automate the amp reduction feature that also might be a work around. I am not knocking the Renology. Two of 40 amp chargers could be ideal in my application.
A VSR would make the reduction feature simple. Power it off the ING. signal and done.
 
A VSR would make the reduction feature simple. Power it off the ING. signal and done.
I thought about that but I need a high signal for the Renology current limiter when the start battery voltage is low. Maybe a VSR in combination with a NC relay?
 
Hi Marinepower.

You may not need a DC to DC converter, with some care you could connect the battery direct to the alternator, have a manual control of the alternator field current to set to safe current values. Whilst this would work, a better solution would be to use something like the Balmar alternator regulator,

I have only had a brief look at the specification but it could be a solution. The main issue with alternators is keeping the temperature within safe limits and the Balmer kit has this feature. I strongly suspect this approach would work allowing the alternator to deliver the maximum current yet stay within limits. An alternative approach is to replace the alternator with a higher output version and/or consider a remote rectifier pack alternator.

With only an hours run its unlikely to cause problems with the lithium pack as the BMS could handle over charge problems. In practice it would be useful to cycle the battery pack well within its capacity so the limits would be not be reached. I dont know if the BMS you are using has the ability to inhibit the charge process below 100% charge. I have used the Victron BMV series battery monitor to limit battery charge at pre programmed capacity levels

Whatever solution you decide to use make sure the alternator is not suddenly unloaded, having the engine starter lead acid continuously connected to the alternator should prevent issues.

Sterling also produce alternator control modules that may be of use.


Mike
 
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A VSR would make the reduction feature simple. Power it off the ING. signal and done.
You have power and ground from the engine side. (Voltage sense) you have a voltage (12 volt) from the Ing. connect the relay side to it and the reduction port and your done.
 
Hi Marinepower.

You may not need a DC to DC converter, with some care you could connect the battery direct to the alternator, have a manual control of the alternator field current to set to safe current values. Whilst this would work, a better solution would be to use something like the Balmar alternator regulator,

I have only had a brief look at the specification but it could be a solution. The main issue with alternators is keeping the temperature within safe limits and the Balmer kit has this feature. I strongly suspect this approach would work allowing the alternator to deliver the maximum current yet stay within limits. An alternative approach is to replace the alternator with a higher output version and/or consider a remote rectifier pack alternator.

With only an hours run its unlikely to cause problems with the lithium pack as the BMS could handle over charge problems. In practice it would be useful to cycle the battery pack well within its capacity so the limits would be not be reached. I dont know if the BMS you are using has the ability to inhibit the charge process below 100% charge. I have used the Victron BMV series battery monitor to limit battery charge at pre programmed capacity levels

Whatever solution you decide to use make sure the alternator is not suddenly unloaded, having the engine starter lead acid continuously connected to the alternator should prevent issues.

Sterling also produce alternator control modules that may be of use.


Mike
Mike, thanks for this. I will have a read through.

Btw: I think the newest fancy regulators are the Wakespeed. Very configurable and very pricey. http://wakespeed.com/
 
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.

I would love to hear thoughts on this from people that are more knowledgeable than myself in regards to doing this.
 
Thanks Grizzman. I did read the manual but missed the part about the 8 volt auto disconnect feature. 8 volts is very low, so perhaps not ideal. If I could figure our how to automate the amp reduction feature that also might be a work around. I am not knocking the Renology. Two of 40 amp chargers could be ideal in my application.
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).
 
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