Can I charge SLA batteries with an Alternator?

[dburlesn]

I have built a remote controlled 21in. lawn mower.
Using a 8HP Electric start engine plus.
Two Motors 24VDC 7 Amp.(No to light Load pulling 3.24 Amp)
I am powering them with Eight 12V 17AH SLA batteries.
Two banks of Four in Parallel - Series.
Giving me 24V @ 68AH
I have a 120v AC SLA charger.
I have fully charged all batteries (One by one, then together in their two groups of four Parallel).<p>I purchased a new 24v 45AMP ONE WIRE Alternator.<p>How can I keep my batteries charged while using them knowing that I'm using 6-14 amps.<p>I really don't want to burn up these batteries!
BUT I really don't want to wait for them to charge off wall current!<p>
Thanks<p>David<p>[email protected]

[k7elp60]

There are two methods to charge SLA batteries. One is the cyclic, in that you charge the batteries at 2.4 to 2.45 volts per cell and at a current of capacity times 0.25. In your case this relates to 28.8 to 29.4 volts at maximum current of 17 amps. Once the charging current drops to .01C or .68amps the charger should be removed or the charging voltage reduced to 27 to 27.6 volts.
The other method is to charge at 2.25 to 2.3 volts per cell at a current limit of 17 amps. In your case the voltage would be 27 to 27.6 volts. In this case the charger does not need to be removed or reduced. This is called the standby mode and the batteries will never overcharge.
All these figures are at a temperature of 77 degrees F.
A good website is:http://www.batteryweb.com/powersonic.cfm
Powersonic has a link to a technical data handbook.
Sometimes putting SLA batteries in parallel can be a bad idea, especially if they are different date codes and have been cycled differently. The reason is that any rechargeable battery that has been cycled has less than full capacity as it is cycled. So over time the full capacity reduses and eventially the battery has to be replaced. When batteries are in parallel and are of different capacities the charging current will vary between batteries, and as a result it is possible to overcharge one or more and not fully charge others.

[ringo47stars]

The problem with charging with your alternator is you have the same charging time unless you switch chargers too. This is after you rig it for charging from the alternator and not using it(battery).

[jwax]

I'm curious, what is going to power the alternator? And what is 8 HP?
John

[sofaspud]

I understand this to be a robotic lawn mower with an electric motor. Without getting into great detail (and in over my head), if the batteries are powering the motor and alternator, it isn't possible to end up with a net gain in energy. In fewer other words, a battery can't power it's own charger.

[dyarker]

I think I figured it out.<p>8HP gas engine with electric starter drives mower blade and alternator.<p>The two 24V electric motors drive the wheels. One motor for each side for tracked style steering.<p>The batteries power the wheels while moving from the garage to the lawn because the gas engine is off.<p>If this is what's going on, then:<p>A 10A (or bigger) diode from battery +24 to +24 connection point. (anode to batteries)<p>+24 alternator output to +24 connection point. Check that when alternator is not turning it is an open circuit. If not, you'll need a 15A or 20A diode here too.<p>When the gas engine if off the battery diode is forward biased, and current can flow to motor controllers. When the gas engine is started, the alternator voltage is higher than battery voltage. The battery diode is reverse biased so no current flows to batteries. The alternator provides the current for the motor controllers.<p>Simple battery recharge:
Parallel the battery diode with a resistor to limit the recharge current. A spec sheet I have for an SLA battery says charge current should be no more than one tenth the amphour rating for battery life. 6.8A here. For worst case let's say the alternator may output 29V, and really discharged batteries 18V.<p>R = (29V - 18V) / 6.8A = 1.6 Ohms A 1.5 Ohm resistor will do.<p>P = (29V - 18V) * 6.8A = 74.8W or more realistic -
P = (26V - 20V) * 4A = 24W So 50W wirewound. The ones in aluminum cases can stand the vibration and being outdoors better than the ceramic core type.<p>That was an example. Recalculate with actual numbers for your parts.<p>This recharge method is slow. Recharging completely drained batteries would take 15 to 20 hours. But with a 5 minute drive to the grass, and an hour mowwing, should be okay.<p>An electronic recharger would be faster because it can keep the current more constant as battery voltage increases (pulse or continueous). With simple resistor, the current decreases as battery voltage increases. When battery voltage equals alternator voltage current is zero.<p>Cheers,

[sofaspud]

Dale, I like your answer a lot more than mine.

[dburlesn]

Thank you,<p>YES from the garage to the lawn.
Start the engine.
The Alternator recharges the batteries.
And runs the motors at the same time?
If I just hook it up will it over charge the batteries.
Do I need to hook up more in parallel?
4 fit great in the battery boxs.<p>The Alternator is a ONE WIRE unit.
All the regulation is built in.<p>The batteries (Mfg. CSB Model: GP12170) are 12v 17AH
I have sixteen of them.
They all came out of a huge battery backup UPS.
From a NASA auction.
Before you all yell at me.......
It was damaged in shipping (BIG Hole in board)<p>Besides, I can say that "NASA" powered my robot lawnmower.<p>Anyway,<p>I'm thinking of terms like a car.
You start it and the alternator recharges that short term drain.<p>I know the alternator will cut back on the current as the batteries get charged.<p>My worry, is that a 45 amp alternator will OVER CHARGE the batteries in the beginning.<p>So what do you think?<p>Want to see it?
How do I add Pictures to a post.
Or just email me.
[email protected]<p>Thanks<p>Again

[dburlesn]

UPDATE !<p>Here is a web page I put together in a rush for you to see the mower, and my progress.
I'll post more pics Thursday (7-8-2004) around 8am est<p>http://www.stuartmperry.com/rc-mower/mower3.htm<p>Tell me what you think.

[ringo47stars]

I'm guessing you probably got a good deal on those sla's if the backup unit was hardly used. But in the current issue of nuts and volts one question was "should I let my nicads R.I.P.". Well sla's have a small number of times they can be recharged too. My advice would be to sell them while they are still good.

[dburlesn]

Sixteen SLA's:<p>Total Cost $20.00<p>He wanted to give them to me, but I insisted he take someting.

[dyarker]

dburlesn,<p>In second post: "My worry, is that a 45 amp alternator will OVER CHARGE the batteries in the beginning."<p>It won't over-charge if the alternator is voltage regulated to around 27.5V - 28V and you use a resistor to limit the charge current (or a transistor charge regulator circuit).<p>-----------------------------------------
Earlier in second post: "And runs the motors at the same time?"<p>The alternator voltage will be higher than the battery voltage when the gas engine is running. So the alternator will supply the current for the motors and recharge the batteries.<p>==============================================
kehearbear,<p>SLAs will last okay (3-5yrs+) if you treat them right:<p>1. Load current about, or less than, amphour number. (check the spec sheet for the type of SLA you're using. They are not all the same! Some are built for high discharge rates.)<p>2. NEVER completely discharge an SLA. For a 12V battery that's about 10V no load. The spec sheet should have a graph of voltage vs current with a line across the curve marking the max discharge point.<p>3. Don't recharge too fast. That causes the batteries to get hot; gas expelled, warped plates, other nasty things. 1/2 to 3/4 of what spec sheet says is max is probably a good target.<p>4. Don't leave in discharged condition for long (days/week/+). Recharge asap. For storage, full charge first, then continuous trickle charge, or full charge at least every two months (more often for sub-freezing weather).<p>5. never, Never, NEVER let an SLA's electrolite freeze. A disharged battery freezes at a warmer temperature than a charged battery. (see #4.)<p>With SLAs with amphour capacity equivalent to a set NiCads, I would expect the SLA to hold up better for high loads. The problem is that the SLA will be larger and heavier.<p>Sample SLA Spec Sheet
===============================================
dburlesn,<p>Does gas engine starter run on 24V, or is there a separate battery?<p>Cheers,<p>[ July 09, 2004: Message edited by: Dale Y ]</p>

[ringo47stars]

Dale Y
Some people don't mind refubishing ni-cads and some people know a simple fix for this is to plug the ni-cad in series with a nickel-metal-hydride while charging it. My point being is that there is alkaline and lead acid they try to refurbish until they realize what happens. Then it is time to switch to something better. I am doing research on refurbishing nickel-metal-hydride and yes you plug it into something in series while charging it.

[dburlesn]

======================
Does gas engine starter run on 24V, or is there a separate battery?
by: Dale Y ]
======================<p>Dale,<p>Yes I am using a standuard 12V Lawn and garden battery for the 8HP Engine.
The engine has it's own charging system.<p>Thanks

[dburlesn]

Update: 7-12-2004<p>I took the mower apart and am making a frame that will allow me to adjust the height of the deck.<p>I took a video of the mower before I took it apart.
I will need to shrink it down before putting it on the web site.<p>http://www.stuartmperry.com/rc-mower/mower3.htm<p>
David