T-Pain Is On A Boat: Re: [Electric Boats] Battery Charging Basics

oak,

It is my understanding that poor regulation may damage sealed LA batteries. The problem is that the burst of voltage up to the top of the sine wave charges the battery, but the drop on the back side of the sine wave causes gassing on the plates. If the top of the sine wave exceeds 14.7 volts, then the more serious problem of overcharging occurs. In the bad old days chargers were called rectifiers because that is all that they did. Back then, all LA batteries were flooded. It was assumed that one would watch the water level closely and compensate for the losses due to gassing. The sealed version of LA batteries is less tolerant of gassing.

Since capacitive filtering is more effective at low current draw than at high current draw and since poor regulation is more damaging when the battery has completed its bulk charging phase, the problem with poor regulation tends to be self correcting. Still, I would make sure I looked at the charger output with an oscilloscope to see that the capacitors are doing their job, especially when the battery reaches about 13.8V to 14.4V (temperature dependent crossover from bulk phase to saturation phase).

Good chargers apply 3 phases of charging to batteries. Bulk charge in the bulk phase applies about 14.4V and constant current to get the battery to 13.8V. Topping charge keeps the voltage high in the saturation phase, but allows the current to taper off to a small value. Float charge drops the voltage to about 13.5V and continues to trickle charge for a long time. Typical times might be 1 or 2 hours bulk, 6 hours topping, and 12 to 24 hours float.

Good luck with the project.

Gary

From: oak

Sent: Monday, November 04, 2013 5:37 PM

To: electricboats@yahoogroups.com

Subject: Re: [Electric Boats] Battery Charging Basics - homebrew chargers

That's basically what I was planning to do.

I'm working with a 24V system, so a little easier....

The "power supply" will basically be an "old school" 24V transformer, run through a bridge rectifier, capacitors, blocking, diode, and 1 ohm resistor (just so I have something to measure drops) - then to a relay.

The Arduino can monitor the voltage of the battery, turn on the relay to supply power for charging, wait for the voltage threshold, and turn off the relay.

The controller can also do nifty things like:

* time how long the charge is taking, and maybe cycle off for a bit if it's been over 12 hours.

* Watch max voltage and turn off if the battery is too high

For first experiments, I wouldn't bother with a "regulator" as such for the power supply - just basically attach it if the battery voltage is low.

There are more advanced things that could be done with a digitally controlled regulator on the power supply. For example, attempt "float" charging at a reduced, fixed voltage - rather than just turning on the supply. But that might be for round two...

The first, most important job for the microcontroller will be to attach the charger, monitor voltage, and disconnect the charger when the battery voltage reaches the threshold.

John

From: Mark F <mark.internet@yahoo.ca>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Monday, November 4, 2013 5:23 PM
Subject: Re: [Electric Boats] Battery Charging Basics - homebrew chargers

Here is my understanding/maybe misunderstanding of a possible homebrew charger.

some form of DC power (solar,a/c with bridge rectifier and caps,etc)at over 56 volts

used as an input supply to the charger.

Using a microcontroller to drive a Mosfet to turn on and off to supply the charger.

Monitor the output pins for voltage and current.

adjust the Mosfet turn on duration to control the current.

Keep the current to within a specified range until the output/battery bank reaches required voltage.

I have been sort of doing this manually.

I have a bank of 40 NIMH cells on my boat.

I have used a Lead acid charger to charge them(48 volt ebike charger)

I have watched the current with a Killowatt meter to make sure the current is not too high.

I have monitored the voltage with a meter.

When the Batteries reach 56 Volts, I remove the charger.

Too say the least this is not ideal.

Any input as to my idea appreciated.

I suspect I am missing something

From: oak <oak_box@yahoo.com>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Monday, November 4, 2013 2:37:06 PM
Subject: Re: [Electric Boats] Battery Charging Basics - homebrew chargers

Mark,

From what I'm seeing, it looks like if you're going to provide a "constant current", you're basically providing a variable voltage supply that responds to the drop across a current limit resistor.

There doesn't seem to be an EASY way to support a wide range of voltage from 36 to 144V.

The microcontroller part isn't so bad. I've used a number of different ones in the past. Latest experience was with the Athena. I think the Arduino was coming out toward the end of when I was messing around with the Athena chips - but they were prohibitively expensive.

At this point in time, the Arduino system has matured a LOT. You can get a basic board for $30. With that, and a USB cable, you can download the IDE and start playing. The IDE (programming environment) is pretty simple to use - I won't say "EASY", but it's much better than I've seen in the past. Just a few quirks. The BIGGEST PAIN I've had with the Arduino is getting new laptops with Microsoft 7 to find and use the drivers. Microsoft - ARRRRRGH!!!!

Anyway, the Arduinos have at least 6 analog input ports (depending on which specific variant you get), and a number of different digital I/O ports.

They have a number of nice built in functions - like floating point math - that makes life simpler for things that you want to be user friendly. These controllers should work very nicely for charging and even battery monitoring.

John

From: Mark F <mark.internet@yahoo.ca>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Sunday, November 3, 2013 2:46 PM
Subject: Re: [Electric Boats] Battery Charging Basics - homebrew chargers

I would like to help, if I can.
I would like to see a charge profile for NIMH batteries as well.
I think it would be good to be able to charge and monitor multiple banks.
It would be good to have a design that supports many voltages, IE 36,48,72,96,144.

I have some experience with microcontrollers from years ago.
What type of circuit would you use for the constant current supply?

Mark

From: Kirk McLoren <kirkmcloren@yahoo.com>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Tuesday, October 29, 2013 12:32:40 PM
Subject: Re: [Electric Boats] Battery Charging Basics - homebrew chargers

hard to get into trouble with a trickle charger.

On Tuesday, October 29, 2013 11:41 AM, oak <oak_box@yahoo.com> wrote:

I hope this will be both fun and educational....

I'm trying to build a small, home brew battery charger. From what I see, it sounds like the primary difference between FLA, AGM, and Li batteries (from a CHARGING) perspective is the charging profile - notably, the voltage level of the charge.

Granted, if you're trying to do a FAST charge with lots of amps, there are a lot more details to worry about. But given the limits of readily available parts, I'm looking at 1-2 amps charging current - so I think I can simplify things a LOT.

It sounds like building a "constant current supply", attaching it to the battery (at 1-2 amps), and monitor the battery till the battery voltage comes up to the prescribed level, then disconnecting the current source.

According to the Battery University (http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery), it looks like I can use a constant current source for the primary charging (stage 1), then maybe connect to a regulated power supply for the topping charge (stage 2), and then use a small microcontroller (Arduino) to monitor the voltage over the next few days / months, and occasionally turn on the "float charge" voltage for a couple of hours. (stage 3)

Does this sound reasonable?

For charging Lithium batteries, is there any difference, other than paying a LOT of attention to the voltage levels for the peak charge level?

From the BatteryUniversity.com site, it sounds like charging Li batteries to 4.1V per cell, and stopping there will extend the life of the battery bank, and provide a bit more guardband to avoid problems.

Thoughts?

Thanks!

John