High frequency ripple and automotive electronics

Thread Starter


Joined Sep 9, 2010
A friend is working on a solar-battery tender for auto, marine and maybe motorcycle applications. Virtually every candidate charge controller involves a switching power supply which will inject some ripple into the system. Is there any reason to be concerned about this?

I tend to think it's no concern since the battery itself acts like a big filter and automotive systems have to be designed for noise immunity anyway. A battery charger is 100% ripple, but of course at low frequency.

Does there need to be a specification on the frequency or magnitude of ripple that is applied to the battery?


Joined Nov 2, 2015
All the chargers I've seen are about 13-15V and some are 16V+ in "jump start" mode. There are a few Technical Service Bullitens about some battery chargers producing more than 15V and its potential to damage control modules. Its hard to find any solid specs on AC voltage. In my experience anything greater than 250mVAC raises a flag. Most chargers have more tham that. I've personally ran a vehicle with a charger hooked up but it was stationary not cruising down the road. Really the only time I deal with ripple voltage it is when you have a symptom of something else drivability wise. You chase it to death only to find its an alternator with a bad rectifier. I have a few examples I could share if you're intrested, but they are not exactly custom solar powered charger related.

Thread Starter


Joined Sep 9, 2010
Maybe a better approach is to just put a filter on the supply and forget about it. With a fast switcher, the appropriate filter should be fairly cheap? It's not worth tons of research – which could never be exhaustive – to avoid spending a few cents on a filter. Just thinking out loud.


Joined Oct 29, 2013
I'm not an expert, but from personal experience it seems that automotive and motorcycle electrical systems are very noisy to start with. Back in the days where car audio was a hobby, filtering noise at audible frequencies was always a challenge (usually alternator noise). Some amplifiers even included features for synchronizing the power supply clocks when using multiple amplifiers to stop them from creating heterodyne noise in the audible frequency range. I don't recall any of these ever affecting the performance of the car itself, aside from dead alternators due to more constant load than they were designed for. Many (most?) motorcycles use shunt type regulators which create tons of noise on their own.


Joined Mar 14, 2008
Since the battery acts as a huge capacitor, I don't think ripple from the charger is a significant problem no matter what the frequency, unless perhaps you are trying to listen the the vehicle's audio system while it's charging.


Joined Apr 28, 2013
Sorry I am not trying to tear you apart here but I need some clarification here.. The purpose of a battery tender is keep the battery charged while the battery is not being used for long periods of time .. So are you using the battery tender while driving or when it's not being used ??

Thread Starter


Joined Sep 9, 2010
.. So are you using the battery tender while driving or when it's not being used ??
I think for maximum safety, yes, we would want to be compatible with using the tender at all times. But it's more likely that the camping scenario would be the bigger concern. Radio, maybe onboard GPS computer, laptop plugged into the accessory port, whatever else you can imagine being used inside a vehicle.

If the switcher is running at say 50kHz, what would a 1A low pass filter require? I have no idea what to specify but let's say no more than 50mV at ≤15kHz. I can't imagine that would cause any trouble?


Joined Nov 4, 2013
So what you are saying is that as of now there has been absolutely no evidence that any filtering is required or that there are not in fact any DC voltage fluctuations or ripple of far higher magnitudes already present in the vehicle in normal operating conditions from the alternator, ignition and fuel systems, fans and or other electric motors in play at any point in time, as in your vehicle electrical systems run at exactly at XX.XXX volts with no more than 50 mV of ripple or noise ever? o_O

Just asking because the reality of your vehicle electrical system, and any device designed to work with any vehicles electrical systems, is designed to work safely anywhere from ~10 volts to 18+volts all with substantial loads of full spectrum electrical noise superimposed on it at any point in time. :rolleyes:

Unless the battery tender has proven, and can be measured, to be itself the continual source of electrical noise problems of one kind or another I wouldn't worry about it. Especially so if it only capable of a few watts of power output to begin with. :(

What I am saying is, it really a "safety concern" or just an exercise in pedantic worrying over nothing of real observable let alone measurable validity? :oops:

Thread Starter


Joined Sep 9, 2010
Pretty much all correct. By safety, I just meant playing nice with the rest of the system. No human safety issue.

I'm in about 99.5% agreement to just ignore this issue unless/until we get into prototyping and prove there's an issue. The remaining 0.5% is thinking I could knock out any potential ripple with about 10¢ of parts, so just do that.


Joined Jan 6, 2014
A bit of noise is probably not going to be an issue, the ignition system on a vehicle creates horrendous amounts of noise as it is, to the point where you can measure the engines RPM from it by dividing the frequency by the number of cylinders. BUT you'd want to make sure that your not exceeding 14.4v on a 12v battery that is connected to a vehicle and take extra steps to mitigate the possibility of an over-voltage fault occurring.

Also (and I know you didn't ask this) you need to think about where the panel is situated, if it's inside the vehicle, then remember that automotive glass cuts UV light and drastically affects the solar panels output.

I did something similar a couple of years ago, with a 25W solar panel and an Arduino based charge controller, but after much tweaking I concluded that solar power was a complete waste of time/money (at least here in the UK).

Thread Starter


Joined Sep 9, 2010
Yeah, the driving force is the possible extension of battery life. It's a lot of cost and trouble upfront for a chance at a few months more life five years hence.


Joined Aug 1, 2013
Depending on the vehicle (and assuming non-diesel), you already have 4, 5, 6, 8, 10, or 12 spark-gap transmitters banging away. If they aren't upsetting your other systems, a (secret make and model) charge controller that has been through FCC certification probably is not going to, either. IMHO 10 cents is reasonable, but 25 cents is extravagant subsidizing of the international capacitor cartels.