Portable AC generator battery charging circuit

[mikefamig]

I've been reading and learning about batteries and charging systems for small power equipment and learned something interesting regarding my 12500 watt Briggs and Stratton portable generator. I'm posting this for anyone who is interested in the topic and for feed back on whether I am understanding things correctly.
 
The first thing that I learned was that the AC output from the generator is regulated only by the engine rpm so it is important to adjust your running throttle to give the desired output of 220 - 240 VAC.
 
Then more interestingly I found that after adjusting the rpm to 3600 as per the service manual and 225 volts at the outlet the battery voltage was 16 vdc. At this I decided to look at a wiring diagram and found that the unit does not have a voltage regulator. It does have a rectifier which is small component which rectifies AC current to DC current but I found no voltage regulator. I can also see the DC battery voltage varies with engine rpm as does the AC outlet voltage.
 
Long story short I called Briggs and Stratton tech support, got escalated to second tier (yes I have too much time on my hands)  and the technician agreed that the DC battery output is regulated only by the engine rpm. So what this means is that the charging system is designed to overcharge lead acid batteries.
 
My question is about this rectifier. I have learned here in the past that a diode will cause a fixed amount of voltage drop so I assume that this rectifier in addition to converting the voltage from AC to Dc is also dropping the AC voltage by a fixed amount from ~120vac to something close to12 volts DC. Close but not close enough to do a good job of charging the battery.
 
So if you have a portable generator I would love to hear what the battery voltage is at full rpm and if you are familiar with a portable generator charging system please enlighten me becasue I find it a little hard to believe that I am understanding this correctly. Why didn't they just put a regulator on the unit?
 
I am considering disconnectiing the battery from the charging system. Then I will take a battery minder, plug it into the AC outlet on the generator and hook it to the battery. Presto, when the machine is in use the battery minder will charge the battery properly.
 
Mike.
 
 
 
 

 

[wkearney99]

I put a battery minder on our Generac.  If only because the default one wouldn't charge while the generator was running.  I can't recall the specifics as to why that was the case.  I discovered after a week-long outage that the battery wasn't charged when it went to start again for it's next weekly exercise test.  I wired the charger into the AC side of the generator's output.  No more battery woes.  (speaking of which, I should probably check when it's due for service...)
 
I'm guessing the omission of a better regulator has everything to do with price.

 

[Desert_AIP]

DC charging off the generator DC outputs is usually really inefficient.
You can get better performance, cleaner power and proper phased smart charging with a DC charger plugged into the AC outlets.
You lose a bit in the AC/DC conversion, but not a whole lot.
You get a lot higher charging amperages too.
 
Depending on the generator design, the DC output may only be going through a diode with no further conditioning, so you get a ripple from the AC, some may even be half wave rather than full wave.

 

[mikefamig]

I understand about keeping the cost of manufacture down but my battery minder only cost me $20 on sale and that is at retail. How much could it cost for Briggs to do something like that? And then they are selling a tool that boils lead acid batteries which is just bad engineering....wouldn't it be fairer to the consumer to just have no battery charging capability at all? If you are going to clain that the tool charges the battery then it should actually charge the battery, not cook it.
 
Mike.

 

[Desert_AIP]

I hear you, I have several of those maintainers around here - generator, lawn tractor, etc.
 
I have a small battery bank, for bulk recharging I have a 24V 25A smart charger. 
It's a bit spendy, but so are batteries (good ones). 
It can tolerate quite a bit of funky input AC and put out very clean DC with the correct charging curve.

 

[RAL]

On the generators I've looked at (and I think this is true for your Briggs generator), there is a separate winding that produces the 12V (approximately) voltage output.  The winding produces an AC voltage, and it looks like a single diode (see page 16 of the Briggs manual) is used to convert this to half wave DC to charge the battery. 
 
Diodes have a small voltage drop (usually 0.4 to 0.7V).  Although that needs to be considered in the design, the main purpose of the diode is just to produce DC, not to adjust the voltage.
 
In my opinion, the Briggs design is a truly crappy charging circuit. It's as primitive as you can get.  If the generator winding produced a peak voltage of no more than 14V, it would be acceptable, but still a poor design.
 
I agree with your thinking.  I would remove the wire from the battery to the charging circuit of the generator, and use the battery minder connected to the Float Charger input as the right way to keep the battery charged.

 

[mikefamig]

Thanks to both of you for confirming my understanding of the situation. RAL- that link that you provided is my generator exactly - Elite model 09801.
 
I bought a Battery Minder for twenty buck around Christmas time at Northern Tool and they want $50 for the same charger now. Now I wish that I had bought a few
 
Mike.

 

[ano]

13.6V is typically the ideal for charging a 12V battery, but chargers drop that down a bit once the battery is charged.  The extra power is dissipated as heat.
 
One interesting thing about the generator is not only does the voltage vary with the RPM, but so does the frequency.  So if you place a heavy load on the generator, you may be tempted to increase the RPM to boost up the voltage back to 240V, but don't because that will increase the frequency to 61 or 62Hz.  It might not make a difference, but it could.

 

[mikefamig]

Take a look at this link for all you ever wanted to know about charging a lead acid battery:
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
 
Quote
 
The correct setting of the charge voltage limit is critical and ranges from 2.30V to 2.45V per cell. Setting the voltage threshold is a compromise and battery experts refer to this as “dancing on the head of a needle.” On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate; on the other hand, over-saturation by not switching to float charge causes grid corrosion on the positive plate. This also leads to gassing and water-loss.

Temperature changes the voltage and this makes “dancing on the head of a needle” more difficult. A warmer ambient requires a slightly lower voltage threshold and a colder temperature prefers a higher setting. Chargers exposed to temperature fluctuations include temperature sensors to adjust the charge voltage for optimum charge efficiency.

 

[Desert_AIP]

13.6V is typically the ideal for charging a 12V battery, but chargers drop that down a bit once the battery is charged.  The extra power is dissipated as heat.
 
One interesting thing about the generator is not only does the voltage vary with the RPM, but so does the frequency.  So if you place a heavy load on the generator, you may be tempted to increase the RPM to boost up the voltage back to 240V, but don't because that will increase the frequency to 61 or 62Hz.  It might not make a difference, but it could.

Inverter generator for all but the heavy 240V loads.