Need to limit DC voltage to 14.8V without loss of current in either direction

Thread Starter

iansl

Joined Dec 6, 2022
10
Hi all,

This is my first post. I hope it is in the spirit of what the forum is for. If not, please feel to delete it.

My problem is this:
I have a motorhome (RV) with a 12V leisure battery. This is connected to an electrical distribution system which charges it at 13.8V when connected to the mains electricity and also takes power from the battery when it needs it to power appliances in the vehicle. It works well.

I also have a separate solar system that charges the leisure battery. This can supply as much as 14.8V which is fine and all works well.

My problem is that sometimes I get a brief spike where the voltage reaches 15V for a brief second and this causes the main electrical distribution system to shut down and sound an alarm - usually at 5a.m.! I suspect it is when there is sun hitting the solar panels and the refrigerator has cooled to temperature and turns off. There is briefly then too many volts and the alarm sounds.

What I would like to do is insert a circuit/device between the leisure battery and the main electrical distribution system that will allow up to 15A to flow in either direction but reduce any voltages that exceed 14.5V on the battery side to be 14.5V on the main electrical distribution system side to stop it cutting off and waking me up!

I don't know if this is possible. I see that ready-made adjustable buck voltage regulator boards can be bought off Ebay but suspect they are not designed for current flow in both directions.

Its a while since I have wielded a soldering iron but am happy to have another go!

Thanks for your time, Ian
 

KeithWalker

Joined Jul 10, 2017
2,616
A diode of suitable current carrying capacity in series with the input from the solar system will reduce the voltage by about half a volt. If that is not quite enough, use two in series.
 

Thread Starter

iansl

Joined Dec 6, 2022
10
Thanks for the reply.

However I do not want to reduce the voltage going to the battery from the solar system. I also dont want to implement a general voltage reduction between the battery and the main electrical distribution system. I just want to limit the voltage to be <=14.5V between the battery and the electrical distribution system whilst still allowing 10-15A of current to flow in both directions.

Thanks, Ian
 

KeithWalker

Joined Jul 10, 2017
2,616
Thanks for the reply.

However I do not want to reduce the voltage going to the battery from the solar system. I also dont want to implement a general voltage reduction between the battery and the main electrical distribution system. I just want to limit the voltage to be <=14.5V between the battery and the electrical distribution system whilst still allowing 10-15A of current to flow in both directions.

Thanks, Ian
Then you are imposing limitations that are causing you a difficult problem. To regulate the voltage between the battery and distribution to a maximum of 14.5V, you would need a supply with a voltage above 14.5V to drive it, which you don't have.
What do you perceive to be the problem with just reducing the solar supply voltage to around 14.5V? That is sufficient to charge the battery.
 

crutschow

Joined Mar 14, 2008
31,126
Alternately, how about a 14.5V low-drop-out voltage regulator from the solar panel to the battery?
What's the maximum solar charge current?

Also perhaps a large capacitor across the overvoltage sense circuit could help.
 
Last edited:

bassbindevil

Joined Jan 23, 2014
640
It would help to record what is actually happening, like with an Arduino set up to monitor several voltages (and ideally currents) and log them to an SD card, or just capturing them on a serial terminal on your laptop. Or point a video recorder at some meters. An analog voltmeter would show voltage fluctuations better than a DVM.
Can you try a different solar charge controller?
Or, perhaps it's just a voltage spike when the fridge compressor turns off. Adding some inductance and/or capacitance might fix that. Like one of those monstrous car audio caps.
 

Thread Starter

iansl

Joined Dec 6, 2022
10
Hi all, thanks for your suggestions.

I have boiled down my requirements into a simple sentence - does that make it any simpler to build?!!

I need a magic box of electronics I could make that freely allows 15A and between 0-14.5V DC to travel in either direction

As simple as that! Hah!!

Thanks Keith, The battery is an AGM (Absorbed Glass Mat battery). To correctly and fully charge this kind of battery, the solar panel needs to throw up to 14.8V at it. The most current I have seen the solar provide is 5A but I think if I lived somewhere a little sunnier it may be higher than that but less than 10A. There is no issue with the solar and the voltage it charges at. This is correct for the battery type so I dont want to alter anything to do with the solar and the battery.

Ian0 Thanks, that looks interesting. I cant claim to fully understand the circuit in fig 10.7. Will that allow high (up to 15A) current flow in both directions while controlling the maximum voltage?

Crutschow As mentioned above, I dont want to manipulate the solar charging aspect as it is working correctly. The capacitor option sounds interesting though although this sounds like it needs a change to the internals of the main electrical distribution system itself. Ideally I was looking to insert a module between the distribution system and the battery to achieve the voltage limitation.

Bassbindevil Is this induction/capacitance fix something that could be inserted between the battery and the distribution system?

Many thanks all for your suggestions.
 

crutschow

Joined Mar 14, 2008
31,126
I have boiled down my requirements into a simple sentence - does that make it any simpler to build?!!

I need a magic box of electronics I could make that freely allows 15A and between 0-14.5V DC to travel in either direction
No that doesn't make it simpler.
You are stating a solution to your problem, not the problem you are trying to solve.
Such a "magic box" could be complicated.
We are making suggestions to solve your problem as simply as possible, not for the one solution you think you need.
I dont want to manipulate the solar charging aspect as it is working correctly.
My and Ian0's solution don't "manipulate the solar charging aspect" they only limit the peak voltage.

And you haven't answered my question as to what is the maximum solar charging current.
 
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bassbindevil

Joined Jan 23, 2014
640
The root of the problem could be that a lead-acid battery isn't good at clamping positive voltage (incoming current) spikes (based on the fact that charging requires a voltage well above the resting voltage of the battery). (I don't think I've ever seen/read a treatment of how a lead-acid battery behaves when rapidly transitioning between charge and discharge, and I don't have enough test gear right now to confirm that experimentally.) So, a cap located at the battery terminals should help, but nearest the distribution thingy would be better. An inductor in the line to the fridge should reduce spikes coming from it.

Or, if you had access to the voltage sensing input to the thing that sounds the alarm... add an RC filter to that so it will ignore brief spikes above 15V.

The closest thing to a magic box I've seen is from Renogy; DC-DC battery charging and MPPT solar charge controller in one box: https://www.renogy.com/dcc30s-12v-30a-dual-input-dc-dc-on-board-battery-charger-with-mppt/
 
Last edited:

Ian0

Joined Aug 7, 2020
6,711
don't think I've ever seen/read a treatment of how a lead-acid battery behaves when rapidly transitioning between charge and discharge,
Look up "Coupe de fouet".
When charging finished, the voltage slowly falls to the resting voltage due to the surface charge. When discharging commences the voltage dips, comes back up again, then starts to fall slowly as it continues to discharge
.
 

Thread Starter

iansl

Joined Dec 6, 2022
10
No that doesn't make it simpler.
You are stating a solution to your problem, not the problem you are trying to solve.
Such a "magic box" could be complicated.
We are making suggestions to solve your problem as simply as possible, not for the one solution you think you need.
My and Ian0's solution don't "manipulate the solar charging aspect" they only limit the peak voltage.

And you haven't answered my question as to what is the maximum solar charging current.
Hi Crutschow, I am sorry I seem to have upset you. I injected a sense of humour into my last reply which seems to have missed the mark in some way. I am very appreciative of everyone's assistance on this forum.

I am also sorry you missed the answer to your question which I had provided....

" The most current I have seen the solar provide is 5A but I think if I lived somewhere a little sunnier it may be higher than that but less than 10A "
 

Thread Starter

iansl

Joined Dec 6, 2022
10
The root of the problem could be that a lead-acid battery isn't good at clamping positive voltage (incoming current) spikes (based on the fact that charging requires a voltage well above the resting voltage of the battery). (I don't think I've ever seen/read a treatment of how a lead-acid battery behaves when rapidly transitioning between charge and discharge, and I don't have enough test gear right now to confirm that experimentally.) So, a cap located at the battery terminals should help, but nearest the distribution thingy would be better. An inductor in the line to the fridge should reduce spikes coming from it.

Or, if you had access to the voltage sensing input to the thing that sounds the alarm... add an RC filter to that so it will ignore brief spikes above 15V.

The closest thing to a magic box I've seen is from Renogy; DC-DC battery charging and MPPT solar charge controller in one box: https://www.renogy.com/dcc30s-12v-30a-dual-input-dc-dc-on-board-battery-charger-with-mppt/
Hi, excuse my ignorance but can you give me a little more info about the capacitor solution you mention? How would I wire this up and what sort of capacitor do you think may be suitable?

For the fridge, I have just googled an inductor and see that it is a coil. Again, I dont know which component to buy and how to wire it so any guidance you can give would be appreciated.

Many thanks,

Ian
 

crutschow

Joined Mar 14, 2008
31,126
I wasn't upset, just didn't catch your humor. :)
The most current I have seen the solar provide is 5A but I think if I lived somewhere a little sunnier it may be higher than that but less than 10A
Then my suggestion is to use a voltage clamp, such as Ian0 suggested.

Here's an example circuit, using a P-MOSFET to carry the peak current during the transient:

The circuit is connected between the plus battery voltage (Out) and the minus connection (ground).

Pot U2 adjusts the clamp point of the output (green trace), shown here clamping at 14.5V. Nominally U2 is set at about 5kΩ.

The transistor can be just about any P-MOSFET rated for at least 20V and 20A.

1670528409564.png
 

Papabravo

Joined Feb 24, 2006
19,592
I wasn't upset, just didn't catch your humor. :)

Then my suggestion is to use a voltage clamp, such as Ian0 suggested.

Here's an example circuit, using a P-MOSFET to carry the peak current during the transient:

The circuit is connected between the plus battery voltage (Out) and the minus connection (ground).

Pot U2 adjusts the clamp point of the output (green trace), shown here clamping at 14.5V. Nominally U2 is set at about 5kΩ.

The transistor can be just about any P-MOSFET rated for at least 20V and 20A.
This is a cool and interesting circuit. If you use it, you should be mindful of how much power may need to be dissipated by MOSFET M1 or any "load dump" devices you may want to connect instead of routing all that power to GROUND. I'm thinking you might want to consider some 100 watt "brown ceramic" power resistors or multiple AO6047's in parallel. This might get dicey if the 5-10A load becomes disconnected. A good design will allow for the unexpected.

Considering the pot has a fairly narrow useful adjustment range you might want to use external resistors and a smaller value pot to limit the range of pot values that you allow. something like 4.5K, 1.5K, 4K will allow you to set 40% to 55%

1670538754440.png
 

crutschow

Joined Mar 14, 2008
31,126
you should be mindful of how much power may need to be dissipated by MOSFET M1
The TS stated it appeared to be a momentary spike so I would not expect much power would need to be dissipated.
sound an alarm - usually at 5a.m.! I suspect it is when there is sun hitting the solar panels and the refrigerator has cooled to temperature and turns off. There is briefly then too many volts and the alarm sounds.
 
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