Thread: DC/DC chargers, How do they work and do you really need one.

[QUOTE=Didge;3001410]

Originally Posted by trout1105

My understanding as a "Layman" abut these units is that with modern (Post 2006) 4WD's the alternator doesn't put out enough voltage to keep the Deep Cell batteries at 100% SOC and the DC/DC chargers can increase that voltage to get the secondary battery/batteries up to 100%SOC........................

What's SOC mean? - please excuse my ignorance
cheers Gerald

State Of Charge in other words how full is the battery

State Of Charge as i under stand it

Originally Posted by drivesafe

................................ That is nothing more than an engineered bogus demonstration.

A bit harsh there Tim, I disagree.


Though you are quite right in noting that many vehicles have forced ventilation through the engine/alternator space which will aid in alternator cooling this is not always the case. Boats, stationary engines, rear engined vehicles and the like do not have this additional (to the internal alt. fan) airflow. One thing the demonstration clearly shows is that the average alternator is a very dumb piece of equipment that will quite happily 'commit suicide' under heavy constant load because it is too dumb to save itself. The video also highlights that with the addition of an external regulator / temperature monitoring, output current regulation can be achieved to optimise alternator output and save an otherwise heavily loaded alternator from failure. Which I guess was the point of the video. I'm not familiar with the Smart Regulator mentioned in the video though Stirling Power have been selling these for many years (in Australia) and they are quite well regarded from my experience.

What wasn't expanded upon in the Victron video though it was noted was that lead acid and lithium batterys have different charging characteristics. Put simply, a lithium battery will suck the guts (current wise) out of the average dumb alternator and do so from 10% SOC up to 90% SOC of the lithium battery. Alternator / battery charging set ups in the average (lead acid battery equipped) vehicle just aren't designed to cope with this prolonged maximum output load. For an equivalent lead acid battery the charge current hits maximum for a relatively short period of time and gradually drops to practically nothing as the battery charges. It takes longer to charge than the lithium but puts less strain on the alternator. If the battery used in the video was a lead acid and not lithium the results may have been very different.

IMO connecting a half charged 300 ah lithium battery (with no BMS) directly to a standard vehicle alternator is not something you would normally do in the real world but OK on the test bench to show the benefits of a 'smart regulator'. Don't do this at home (or in your own vehicle) folks

Lithium batterys make great house batterys. They're light, have good power density, relatively constant voltage, are quick charging (with the right gear) and can withstand comparative deep discharge better than lead acid. But one thing they don't do on their own is manage / limit charge current. When you buy your typical lithium drop in replacement battery the spec sheet will tell you all sorts of wonderful things about its internal BMS, voltage regulation, low voltage cut off etc. but nothing about (charging) current regulation (because it doesn't have any) and the risk of burning out your alternator. With multiple batterys the risk increases.

Getting back on topic, this is one application where a DC-DC charger really excels.

In our Oka camper we have the factory fitted battery(s) ie. 2 X 12v lead acid batterys, one is a dedicated start battery and one 'vehicle' battery both charged by a very generic Bosch 14/85 alternator, ie. 14v @ 85A. The house battery is 2 X 100 ah LiFePO4 batterys of N70 'drop in' type. There are two separate charging sources for the house battery. One charging source (depending on weather ) is 2 X 220 watt roof mounted solar panels connected directly to the battery via a Votronic solar reg (which has specific LiFePO4 charging profiles) which can be manually switched in or out. The other charging source is the vehicle alternator whose output is connected via a 40 amp Votronic DC-DC charger which also has selectable LiFePO4 profiles and can be switched in or out as reqd.

There are three main advantages in using a DC-DC charger to charge the house battery.

1/. It has specific lithium battery charging profiles to ensure optimal charging off the battery.

2/. It limits the lithium battery charging current to a maximum of 40 amps or less than half the rated alternator output leaving enough current / power for the rest of the vehicle and ensures the alternator is not overloaded. It might take longer to charge the house battery than could be the case but 'fast charge' is not important to me.

3/. It provides isolation between the house battery and the vehicle battery(s). This is quite important as not only does it stop the house battery back feeding the vehicle it allows both house battery charging systems, solar and alternator, to be used at the same time. One of the problems that can arise if both solar and alternator outputs are connected directly to the same battery is that the solar 'boost' voltage can back feed to the alternator whose voltage regulator will sense 'over voltage' and shut down the alternator. Not a big deal in a non electronic dinosaur like the Oka where the alt. lamp flashes and the tacho stops working but I'm not sure how this would affect a more 'modern' vehicle.

Deano

Speaking of boats I have seen this posted on a few forums re Etec outboard that run alternators...

The alternator in that vid was free standing and not closely mounted to a hot engine, I would hazard to guess that the alternator in that vid would run a lot cooler than and alternator bolted to a running engine regardless of how much airflow was introduced to it especially at low revs/road speed.

I have first hand experience with charging lithium batteries directly from the alternator and while the system had to be changed, the issue that required the change was to do with continual discharging of the lithium battery back through the lead acid cranking battery.

This setup was in a D4 and carried out about 7 years ago.

There was a 80Ah lithium in the auxiliary battery tray and was charged directly from the alternator, via one of my isolators.

The system ran like this for about 12 months until it was realised that because the lithium had a settled voltage of 13.3v and the cranking battery had a settled voltage of 12.7v, once the motor was turned off, the lithium continually slowly back discharged into the cranking battery.

This was a learning curve for both the D4 owner and myself.

This setup not only continually discharged the lithium while the motor was off, but also meant that as a result of the continual discharging of the lithium while the motor was off, EVERY TIME the motor was started, the alternator was run at full output current to recharge the lithium and cranking battery.

Going by the claims in that video, this would be a worst case situation yet no mythical failure occurred.

The owner replaced the lithium with an Optima, and moved the lithium to the rear cargo area.

His current setup is two 180AH lithium batteries mounted in the rear cargo area and when needed are charged via an Ignition controlled relay, allowing charging direct from the alternator.

Again, no alternator vaporising when he charges the lithium’s.

He also tows a large caravan which has four 100Ah lithium batteries charged via a DC/DC charger.

So why is he not replacing his alternator every time he goes on a trip?

ummm,

that videos bias is frightening, but Im not sure if its intentional or ignorance.

Originally Posted by DeanoH

A bit harsh there Tim, I disagree.


....

Nup! I also have to agree with Tim on that 'quality' of that video demo.
Did you notice the model of alternator they used as the burn out mule?
the cryptic code numbers(14V70A) on the alternator body gave away a bit of a clue.
70A rated alternator for a few model of vehicles, but that specific one obviously Citroen(Valeo A11V1)
The specific car is of no consequence tho, but the 70A rating is!

If you run more than 70A(they drew 79A at one point) out of a alternator designed to output 70A, you'd fully expect it to burn out at any revs out in the wild, let alone low revs in a lab environment where airflow didn't seem to be a priority.

Had they simply added the external regulator on the Citroen alt, and limited it to max 70A, or used the Lithium BMS appropriately, would have resulted in a higher probability that the alternator would have survived for the duration of their experiment.
From what I just researched of the Balamar alternators, they come in a few varying sizes 70A, 100A, 120A and 150A.
You would have hoped that they'd used like for like gear, so the 70A model, but the fact that they got 94.4A at 3600RPM, would indicate that they may have used the 100A model.
Would that be a fair comparison?

Simply a case of engineering mismatch using equipment not designed for the purpose it was intended too verses higher rated equipment.

So Tim's comment wasn't harsh at all.