Project Float Charger

I'm interested in doing a project both for the practice and to be useful. It would be something like a Battery Tender automatic float charger.

The basic idea is a microcontroller to control based on battery voltage and current. If I understand correctly, you charge at maximum current until you reach 14.4V on a 12V battery, then you lower the current to prevent the voltage from going over 14.4V. After the current reduces to perhaps 0.1A, the charge is terminated and float mode keeps the voltage at 13.2V, if the voltage goes under 12.6V, the charge cycle is restarted.

My question is what to use to interface the microcontroller to the battery? Voltage regulator circuit or what. I need to sense voltage and current and control the output. I can come up with something to work but I'm looking for ideas that might be better than mine.

RogerN

Reply to
RogerN
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What you describe below is not a float charger, so you should change the subject line to battery charger.

Do some more research on battery charging. You don't charge at maximum current, you limit the maximum current.

If you're talking about a lead acid type battery, you charge it to ~14.4 volts, then derop the charging voltage. In any event, the .1A figure is meaningless without knowing the battery capacity as well as type.

(Again, lead acid assumed) 13.4 or 13.5 is a better target. 13.2 is a bit too low.

That ("charge cycle") means you are not float charging the battery.

You really need to specify a proper charging scheme before worrying about a ucontroller.

Ed

Reply to
ehsjr

On a sunny day (Mon, 22 Feb 2010 21:16:20 -0600) it happened "RogerN" wrote in :

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Reply to
Jan Panteltje

I got the information from Maxim IC and Battery Tender websites.

I meant the maximum rated current for my battery charging circuit, I will probably have a 1.5A charger so my maximum current will be 1.5A. I guess I would prefer to call the stages current limited, voltage limited, and float.

My info was from Maxim IC and/or Battery Tender website. I thought possible loads on the battery could als prevent the current from ever going to the full charge state.

That is a minor software adjustment, I also plan to sense temperature and adjust the voltages accordingly.

This is a microcontroller project, once the hardware is in place I can make software changes to get the charging scheme right.

RogerN

Reply to
RogerN

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You don't really need the micro but if you really want to use one then:

A PWM output to control the switching transistor in a buck topology regulator is likely already in the micro. You want this buck circuit to work correctly all the way down to zero volts on the output terminals. This means you need more inductance than you would for the normal case.

You need to be sensing the current going into the battery. A simple resistor will do for this but the circuit must work with the output shorted to ground. You want this sensed current to provide instant feedback that shortens the on time when the limit is hit. You also want the micro to be able to see this current value on a ADC.

The terminal voltage of the battery needs to be measured by an ADC. This needs to be very accurate up at the full charge voltage. It doesn't need much accuracy at low voltages.

Measuring the temperature of the air or better yet the battery can help you to get the right charging voltage when the temperature is unusual.

Ideally you will have a MOSFET between the charger and the battery that isolates the battery from the charger when the power is off. You don't want the battery to be discharged if there is a power failure or your system gets unplugged.

Reply to
MooseFET

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Beware! When you design a battery charger, someone will always connect the battery backwards.

Reply to
MooseFET

That's oddly stated. Normally a lead-acid battery charge is described as "constant voltage with current limiting", and for all but absurdly large chargers the current limiting is there for the charger, not he battery. The _behavior_ is the same, but the normal description captures the fact that the _important_ part is the constant voltage.

Note, too that the optimal charge voltage varies by battery temperature. I don't know how critical this is, but my understanding is that if you're going to be charging in an unheated space you want to monitor at least ambient temperature and adjust the charge voltage accordingly.

As mentioned, this magic current depends on the battery capacity to a great extent, and (probably) to a lesser extent on the battery construction.

That depends on how much work you want to load onto the microcontroller. You could just have a current-limited voltage amplifier (i.e. a microprocessor-controlled regulator) that you set and forget, or you could drive the base (or gate) of a transistor directly from a pin on the micro and let the micro do _all_ the work, or you could go somewhere in between. It depends on your abilities, and what you're trying to do.

There's are books on the care and feeding of batteries -- you may want to get one. When I get downstairs I'll try to remember to send you info on the one I have.

--
Tim Wescott
Control system and signal processing consulting
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Reply to
Tim Wescott

On a sunny day (Tue, 23 Feb 2010 06:44:10 -0800 (PST)) it happened MooseFET wrote in :

True :-) So when charging a battery add a diode.

Reply to
Jan Panteltje

:)))))))))))))

Nope.

VLV

Reply to
Vladimir Vassilevsky

On a sunny day (Tue, 23 Feb 2010 14:56:50 -0600) it happened Vladimir Vassilevsky wrote in :

Why not? Just set the voltage .7 V higher.

Reply to
Jan Panteltje

Because the current will flow from the battery into the ground through the inductor and diode in your circuit and back into the negative of the battery. PDQ something will smoke.

Reply to
Raveninghorde

Current flow with reverse battery is in the same direction as charging current. Series diode just stops battery powering up the charger.

Commercial battery chargers will sense battery voltage then close a relay it the battery is right way around.

Another option is a reverse diode across the battery terminals to either blow a fuse or have a polyswitch or lamp to hold back the current.

Battery voltage sensed via diode can be sloppy due to diode's varying voltage drop, but might be okay for float use.

Grant.

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http://bugs.id.au/
Reply to
Grant

On a sunny day (Wed, 24 Feb 2010 09:57:25 +1100) it happened Grant wrote in :

Oops, you are right! Thank you for pointing it out. LOL, I should know, did that reverse protection thing with a thyristor charger.

So what happens when you connect the battery the right way, the relay comes up, then you disconnect the battery, but now the relay stays up, as the charger makes output voltage, and then reverse connect the battery? ?

Reply to
Jan Panteltje

I would pull the output up to +24V with a resistor so when the battery is removed there is a clear signal. Detect +24V and drop out the relay.

Reply to
Raveninghorde

Relay drops out as soon as charging current collapses or voltage below minimum for battery size; for example, disconnect at below 16V for a 24V battery.

Grant.

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http://bugs.id.au/
Reply to
Grant

I was wondering about using a linear voltage regulator to scale the output, perhaps 0-5V from the microcontroller could result in up to 15V output from a regulator (or amplifier circuit?). I thought perhaps PWM out to a R/C, switch the OUT pin to IN , read voltage and current, calculate new PWM value, repeat.

I could buy a Battery Tender but I could use several and a microcontroller project sounded fun. Plus I want to be able to do some things that the $50 Battery Tenders don't do.

RogerN

Reply to
RogerN

My Battery Tender was a useless POS, never worked correctly. I just threw it out for the 2nd time. Dad saw it in the bin at the curb this morning and brought it back. I *hammered* it this time, and dropped it back in the bin for next week's pickup.

Reply to
Beryl

You could. An amplifier with a pass transistor will work well; there are slews of batteries that you can charge from a 12V rail.

Lots -o- work. I'd do a web search -- I seem to recall peak detection chargers in QST and Circuit Cellar, which means there's been some in Volt as well.

--
Tim Wescott
Control system and signal processing consulting
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Reply to
Tim Wescott

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