Myth Busting: Charging lithium via alternators

Please keep in mind that every vehicle is different, especially if it's a custom converted van or DIY van. The information below is based on 76 years of battery market experience, along-side lithium battery engineer's instruction, but we are not auto-electricians so we cannot give guarantees on your specific vehicle. You may wish to get second opinions, or a professional to inspect your components before making a decision based off this post.

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Looking for the fast answer?

• DC to DC chargers are preferred by us as they give a proper charge profile that makes for efficient charging, but split charge relays are the next best thing and can still be used with our lithium batteries.
• We do not recommend to charge any lithium (not just ours) directly from the alternator; there should be a diode, relay or DC to DC charger connected in-line.
• If your vehicle does not have a split charger, and you want to charge the lithium battery via the alternator; we recommend installing a DC to DC charger over a split charge relay.
• On marine craft that has a direct feed from the alternator to the lithium battery, we strongly recommend installing a DC to DC charger, or at least use a split charge relay and connect with the engine or bow thruster battery.
• There are some alternator exceptions that work without issue when connected directly to lithium - it's not possible to always verify what alternators can do this, so as a business, we have to make a generalised recommendation to the market.

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Like many of the questions we get asked, our answers usually end up depending on your wiring, accessories and charging equipment - but when it comes to alternator charging, how are you supposed to what alternator you have? How does your factory fitted alternator work, what's it idle running speed, does it have internal fans? These questions are near impossible for most users to answer, so it makes it very difficult to make an informed decision on whether to install a DC to DC charger or not, and also makes it difficult for us to give recommendations. At TITAN, we've yet to hear about a alternator problem resulting from a lithium battery installed - we only know about the online discussion. Many of our batteries have been installed into vehicles that are set-up for lead-acid and have been operating perfectly for years; our batteries are designed specifically to encompass leisure vehicle use and to be installed without new equipment.

Many lithium brands, who also sell DC to DC chargers, will state that you can only use their lithium batteries on a vehicle that has a DC to DC charger installed. Some even make videos to prove their point:

There has also been reports that alternators go up in smoke in various forum posts, so it all very much seems like you should definitely install a DC to DC charger if you purchase a new lithium. Much like our answer to other questions, we say that it depends.

It's worth remembering who exactly is giving these recommendations and where they come from - most companies buy in batteries, re-brand them and then basing their instructions from the original battery manufacturer, or lab tests like the video above. Our recommendations come from real-world working knowledge and 76 years of it.

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What causes overheating?

As stated by Victron in the video above, and is generally the accepted reasoning, the issue is due to lithium having a very low internal resistance. This low resistance means that it can accept a high current charge with very little heat loss which makes them fantastic for charging fast. However if an alternator is running at low RPM, so it's internal fans are also running at low speeds, there is a case to be make that it will still try to output it's full current rating, as it cannot see the higher resistance of a lead-acid battery. Lead-acid batteries do have some resistance, which varies depending on the state of charge - the alternator can see this and will output accordingly.

One big thing to note here is the alternator in the video above is a standard petrol car alternator rated at 70A and is being run without cooling, is indoors, is not connected to any common devices (like a split charge relay) and run at a low idle RPM. The battery is also not controlled with a BMS (Victron sells their lithium batteries without a BMS - you have to buy it seperate); something that should always happen when charging - all of these conditions are not found on alternators connected to vehicles; and all leisure vehicles are converted large vans, that will have larger, beefier alternators that can output 150-200A. It's a little different when it comes to marine use, as 70A is generally the size that is used, and would be inside a engine compartment with little airflow - we'll get into that below.

It's also important to note that Victron aims many of its products towards the marine market, so this video doesn't fit too well in the leisure (caravans, motorhomes & van conversions) market too well. It's also important to note that there is monetary value in videos like the above; these are big companies that will happily sell you extra charging equipment alongside their batteries fuelled on customer's safety fears - whether true or not.

Alternators will spin faster than the engine RPM; usually at a 3:1 ratio. They have in-built fans as part of their construction, so when you are travelling to a site, the internal fan will be spinning at a high enough level to keep the alternator within its operating temperature range. You will also have engine bay fans that cool the whole bay - the alternator is typically located near this fan too so will have plenty of airflow to cool. The RPM of the alternator while the vehicle is idling is usually still fast enough for it to cool itself, plus most journey's idle time is considerably lower than the time spent at speed.

These are some of the reasons why we have never seen an alternator problem with a lithium battery installed.

What about diodes?

There's another reported issue with charging from an alternator with a lithium battery BMS that suddenly shuts down the charge flow once the battery reaches 100%, or stops the battery from charging for another reason. This sudden shut down of current flow can cause damage to the alternator diodes and wear them out faster than they should.

This is one we are more focused on when it comes to marine users as there's a higher chance that vessels may come without split charge systems, and is why we do recommend marine users use a DC to DC charger if charging their house batteries directly from the alternator. Many current DC to DC chargers can be paralleled in order to give higher charge currents, so users can still enjoy fast charging.

However, almost all leisure vehicles would have a split charger installed as it's standard practice. Split chargers are relays that wait until the main battery (typically the engine battery) reaches a certain voltage level, then will switch to allow the alternator current to flow to both batteries. This essentially connects the engine battery to the lithium battery while charging and is why we say it is okay to use out lithium batteries with a split charge system. Because the batteries are connected together, the issue of resistance is no longer a concern as the alternator is always seeing the lead-acid starter battery level. On top of this, the lead-acid can be seen as a dump or sponge to soak up any excess - also nullifying the diode problem; if the lithium shuts down it's own charge function, the current will go straight to the engine battery, rather than back to the alternator or diodes. Lead-acid batteries are more forgiving when it comes to short peaks in voltage or current.

Closing Point

TITAN BMS has the highest spec of BMS installed in lithium batteries today - we ensure our current allowances can handle today's common charging practices, and have developed it with a focus of ensuing there are no safety or damage concerns when installing our batteries.

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For some more compelling evidence from an independent channel, please check out this video from On The Road With Ron who does some real-world tests:

*we aren't affiliated with Ron or his channel - his video is one of many we came across while researching this subject.