Alternator Voltage Boost Modification

[luthj]

This technique will work on most any alternator. I did it to a Bosch Internal fan type alternator, which is a very common model, used in cars, trucks, and some heavy equipment.

Some background reading

http://www.smartgauge.co.uk/alt_mod.html

http://what-when-how.com/automobile/...on-automobile/


So, what is an alternator? In short, current flowing through the spinning field coil (the rotor) creates high frequency AC power in the stator (stationary) coils. This is rectified to DC by a series of diodes, which are mounted behind the regulator and cover. Control schemes vary, but all alternators regulate output voltage by varying current in the field coil, which if fed via 2 brushes and the slip rings.

The current is varied to maintain a specific voltage setpoint. On 12V systems this varies from 13.3-14.7 volts depending on vehicle type and temperature.

The regulator on these Bosch alternators compares the voltage between B+ (alternator output) and ground to its reference voltage. The current in the field is adjusted accordingly.


How does this modification work?

Commonly known as the diode mod, it has been used for decades in marine and automotive segments. By adding a diode to the B+ side of the regulator, we fool the regulator into thinking the main bus (B+) voltage is lower than reality. The regulator will then adjust the output voltage so that it is equal to its internal setpoint +Vfwd of the diode.

Is it safe?
Done properly there is no risk to the alternator. The total load, and max output amps will not be affected. If the diode fails, the regulator will loose power, and the alternator will shut down gracefully.

Why is this needed?
Some vehicles (such as my sprinter) have alternator output voltages when hot of around 13.6V. Add in some cable losses, and this is much to low for effective charging of lead acid battery banks. Boosting the voltage not only dramatically speeds up charging, it will make for a much longer lasting battery bank.

Other considerations.
Excessive voltage can damage vehicle electronics. Most vehicles are safe up to about 15V. But do your homework.

High charging voltages can cause damage to some sealed batteries, so take the type/design of all batteries in the system into account when selecting a diode(s).

 

[luthj]

The mod.

This link covers the removal of the regulator from Bosch internal fan alternators.
http://www.myturbodiesel.com/wiki/alternator-pulley-removal-and-low-voltage-troubleshooting-vw-audi/

Here is a pin-out of the regulator used on these units. Note that the connector in the upper right will vary by model. On my sprinter it is a single stud.



Here is a photo of my regulator with the PCB exposed.



The goal here is to insert a diode between the ring terminal B+ and the regulators PCB. Given the small leads and the cooling needs of the diode, I opted to cut the ring terminal off, and route it outside the alt case.



I trimmed the ring terminal down, and soldered a lead to it. The ring terminal at the end of the break-out harness is attached to either of the two B+ terminals on the alternator. These are the big studs above the regulator. On my vehicle one of these is unused, so it was easiest to attach it there.

I chose to use 2 diodes wired in reverse parallel. On some alternators, there is the possibility of a voltage spike with a single diode, so having two wired as such prevents that possibility.

The two spade connectors on the break-out harness are for a 12v relay. The relays normally closed (or open in preferred) contacts bypass the diode. This allows me to enable the "boost" function with a dash switch.


Other options:
If you have the alternator on the bench, is to install the diode under the back cover. There may be enough room for this. Take note that some air cooling flow will be needed for the diode.
The ring terminal can be bent straight up. After some trimming it can be made into a male spade terminal. With a hole cut in the plastic back cover, a female spade connector can be connected. This would make installation solder free, and allow for easy swapping of diodes to change the voltage boost.

Selecting the Diode
This smargtauge link goes into detail on diode selection.
http://www.smartgauge.co.uk/alt_mod.html

The diode will determine the size of the voltage boost. More than one diode can be used in series to increase the voltage boost. The forward biased voltage drop Vfwd will be the increase in alternator output.

My alternator is a 200A version, and thus the field current will peak at around 5-8A. I chose 10A schottky diodes which are commonly used in solar panel blocking. Their Vfwd is about 0.55V, but will vary with current and temperature somewhat.


The Results

My sprinters alternator voltage generally started at 14.3V when cold, and tapered down to about 13.8V hot. In very hot temperatures this could be as low as 13.6V.

After adding the diode, cold startup voltages are 14.7-14.8V, and taper to about 14.3V when hot. Much improved.


Final thoughts.
This mod should work on both digital and analog alternators. Older analog controlled alternators use a separate D+ output to generate power for the regulator and field coils. This is not applicable to the alternator I modified. If modifying this type of alternator, the diode can be inserted on the regulators ground leg, or the D+ lead to the regulator.

 

[john61ct]

I would have been skeptical but Gibbo, inventor of the SmartGauge BM, is a super-knowledgeable authority.

Just for completeness I want to point out the "conventional overkill" alternatives for those with a very expensive House bank that needs a more precisely regulated charge cycle:

Convert your alt to a Balmar MC-614 external voltage regulator, connect its output directly to your House bank, and use an inexpensive VSR/combiner or Echo Charger to keep Starter topped up. Or,

Put a DC-DC charger between alt (and any other "rough output" charge source) and your House bank. Sterling's BB series is my preferred make, with ampacity up to 120A, 180 & 240A coming RSN.

Both fully user adjustable setpoints and thus suitable for caring for LFP.

 

[luthj]

The external regulators are a great option. They will compensate for drops in the wiring and battery temp. They are a bit pricey. Also, the bosch alternator I am working with actually limits alternator output based on RPM in order to prevent overheating. It may have a temp sensor in the regulator circuitry as well. An external regulator may need to be output limited in some applications.


The dc-dc chargers are great pieces of kit. Not cheap, but neither are batteries.

With regards to LFP most don't appear to need a special profile. Instead charging to the 90-95% SOC votlage (around 14.1v?) for many prismatic cells gives longer life and prevents overcharging. Of course a safe to charge and possible high SOC disconnect relay may be called for. LFP do like some temperature compensation, but I am not sure how many chargers will let you manually set the temp compensation coefficient.

Give how quickly and readily LFP charges, even at lower voltages, combined with its love of partial charge states, I would think most LFP banks could be charged directly from the alternator at 13.8-14.2V without any additional regulation. Other than a charge disconnect relay for high SOC or voltage.


Depending on the size of the LFP bank, limits on charge current may become an issue with larger alternators. So a dc-dc charger may be called for in those cases, simply to prevent exceeding the recommended max charge rates.

 

[john61ct]

I use 13.8V (3.45Vpc) as a hard maximum for LFP for maximum lifespan. Zero Absorb time, ideally no Float either, and as you say don't fill up until you have loads ready to draw back down.

Temperature compensation is so close to 1.0, best to just turn it off.

But as long as temp **protection** is in place and wiring is robust, very high currents, fully recharging in under an hour is no problem at even hundreds of amps - assuming the BMS allows it.

MC-614 VR has great current limiting features to prevent that high CAR from frying the alt, or of course a DCDC charger will do the same.

 

[luthj]

Good info, thanks. It is surprising how many LFP cell manufacturers quote a 14.6V charge voltage, without caveats that this the absolute maximum...

With the cloudy and short days in NZ currently, I am finding a LFP bank more appealing. Getting a full absorb cycle is freaking hard without shore power. Hence my recent alternator modification.

 

[john61ct]

My theory is, for some reason they don't want us to keep our banks going for decades

Actually, 99.9% of LFP usage is much higher C-rates where they'll only go a few thousand cycles anyway and range / capacity counts for a lot.

But gentler House bank usage with lower voltage charging I think we'll see 3-5x the spec'd lifetimes. But early days yet, and word of mouth only from users, no official sponsored research.

 

[john61ct]

And of course since consumer's gear is oriented to lead, they want to promulgate the "drop-in" myth for marketing purposes.

 

[luthj]

Other than cycle count, a major deteriorative factor in LFP is anode breakdown. This is dramatically accelerated by high SOC and higher temps. Not really a big deal on frequent and deep, and/or high rate cycles, but when running at fractional C between 50 and 80% SOC, it can cut life in half. I will have to dig around, but I did see one guy who had a cycle tester going on a thousand cycles on some LFP prismatic cells. He was seeing results at 900 cycles that indicate a 2000 cycle (to 80% rated) capacity was reasonable. With a 60% rated capacity replacement expectation, you could get 10 years or more with daily usage at 80% DOD, but with lower discharge rates, and only charging to 90%, in theory 5-6,000 cycles, or 15 years is possible. The real question is whether the internal cell degradation will kill the pack first. Lower pack temps, and staying out of the charging knee are probably big factors.

https://marinehowto.com/lifepo4-batteries-on-boats/

 

[john61ct]

Yep Maine Sail is da man, everything wrt electrickery at that site's worth reading.

Actual capacity sacrifice stopping charge around 13.8V is nowhere near that much, still within rated AH on most good vendors' cells I've worked with.

Just so pricey, a very long ROI compared to lead, which makes it risky for most.

 

[luthj]

Yeah cost is the killer. I have been seeing some used tesla packs for sale for very good prices... Lithium cobalt is another game entirely, but if the prices keep coming down... A man can dream.

 

[john61ct]

Well solar's come down so much, for many it may come close to balancing out.

Keep in mind a 600AH lead bank can be replaced with a 400AH LFP one.

And forget used stuff, EV packs and other lithium chemistries, strictly for hobbyists doing science experiments, too dangerous in a mobile setting.

 

[Rando]

If you are on for a project, you could consider a DIY LiFePO4 pack from surplus "D" cells:
http://www.wanderthewest.com/forum/topic/15342-low-cost-diy-lithium-camper-batteries/

I built a 150Ah pack this way for ~$300, and it has been working beautifully.

 

[john61ct]

Such deals are pretty rare, counterfeits abound, and personally I'd rather just get 4x or 8 high capacity prismatic cells from a known-quality US or European vendor.

Link to the BMS you use?

 

[Rando]

The batteries I got meet spec, so I am happy with them. The same seller on ebay still has some of these surplus cells, but it does appear that his supply is running low.

Secondly, all retail LiFePO4 batteries are somewhat grey-market chinese products, so for 1/3 the cost of other options this was a great option. The BMS is a generic from Battery Space, similar to many available on ebay/amazon.