Recharge SLA battery without disconnecting from Robot?

[SeagullOne]

I have a 12V 35Amp Hour Sealed Lead Acid battery on my robot. The battery charger that I got for it recharges it well, but I've just been disconnecting the battery from the robot each time and using the alligator clips.

Now, the battery charger I got also appears to have another apparatus: it looks like two ring terminals, one red and one black (for + and - ) a 15 amp fuse box, and another plastic wire connector that meshes with one directly on the batter charger. I'm assuming its for charging the battery with the charger without having to disconnect the battery from the electronics. The ring terminals are already connected to the battery terminals: just plug the battery charger into that little wire connector and apply power to charge the battery, right?

Here, here's my main question. Could I charge the battery like this when the robot is powered on? I'm not going to explode the battery, right? If power is going through the terminals, where would the power from the charger be going?

[Soeren]

Hi,

Not exactly sure I understand your question correctly, but to give you an example with a battery, a load of say 2A and a charger able to deliver 5A, it looks like this:
The load sucks the 2A it needs from the charger and since the charger can deliver 5A, there's 3A for charging the battery (I_Supply - I_Load).

That said, be careful not to remove the terminals from the battery while the charger is turned on, because you cannot make a clean break, so the process will generate spikes that could potentially kill your electronics.
To guard against that, an LC-circuit followed by a MOV/Tranzil/Tranzorb/whatever could be employed.
(A pair of fuses should sit near the battery where it connects to the electronics, as well as to the charger, since your battery would be quite an annoying acquaintance, should you shunt it un-fused).

Before seriously putting the concept to use, do make a full charge while monitoring voltage and current, especially at the point where it is near a full charge.
Your battery should never see more than 14.1V and charging current should be from 3.5A (min.) to 12A (max.) initially - it's really important to keep the initial current within these limits for keeping the battery happy!

Best thing would be to make a small charging circuit for mounting permanently at or near the battery and then have a connector on the 'bot/ROV that can be fed either AC or DC (or, if space allows, rebuild a small PC supply and just plug your vehicle into the mains).

Did that answer your question?

[SeagullOne]

Thanks for your advice, Soren!

The directions say to keep the charger far away from the battery as possible, so I'm hesitant to mount the charger on my robot. However, attaching a charging circuit, like you mentioned, seems very similar for what the apparatus does.

Once again, the apparatus is just two ring terminals (that connect to the battery), some wire with a fuse box mounted with a 15Amp fuse, and a plastic wire connector that can be plugged directly to the charger mechanism. So, what I could do is keep those two ring terminals attached to the battery permanently. Whenever I want to charge the battery, I just plug the plastic wire connector into the charger and apply power to the charger. That way I wouldn't have to take off the duct tape and put on the alligator clips every time I want to charge the battery.

By the way, the SLA battery only powers the motors through the Sabertooth 2x25...so I would normally have the kill switch off when I'm charging the SLA battery, since I don't want the robot to go anywhere anyway. But if the kill switch is on and power is going through the battery to sabertooth, it'll likely overshadows the 2 amps coming from my charger anyway (with the motors running). So that said, assuming my battery didn't totally explode, the worst I can expect is my robot to just rip itself away from the power socket! 

I would just have to worry about a voltage spike damaging the electronics of the Sabertooth should the robot really pull itself out the socket while power is going into the Sabertooth.

So assuming the way you explained things, Soren, the battery shouldn't explode. That's a relief! If the motors are running and its pulling more than 15amp from the charger, the fuse on the apparatus will go out and the battery charger isn't damaged and no fires start. (I also have a fuse on the robot).

Is all this right?

[want2learn]

Something to watch for when charging lead acid batteries is hydrogen disharge when charging (mainly overcharging). Even sealed lead acid batteries discharge hydrogen when charging.

Don't believe me? why do they have vents?

Just make sure the gas can't build up inside your robot!

[SeagullOne]

I know, want2learn. I know. 

The battery is well ventilated on my robot. Thanks for making sure with me though.

[Soeren]

Hi,

The directions say to keep the charger far away from the battery as possible, so I'm hesitant to mount the charger on my robot.

Conventional (valve regulated) lead-acid batteries, when charging, lets out oxygen and hydrogen, which together is very explosive, so you don't want to risk producing a spark in the vicinity of a charging (or newly charged) battery - that's why you should allways turn off the charger before removing the chargers cables form the batetry terminals, when charging an automotive battery with a mains operated charger.
In the SLA, these gases are recombined (into water) inside the battery and explosive gas will only escape, through the vents, if charged with a too high current or to a too high voltage.

When the charging circuit is mounted permanent to the battery, no sparks are produced in the danger zone, since no electrical connections are broken near the battery, so there is no problem of that kind.

Another possible reason could be, that the charging circuit would drain the battery or be destroyed by reverse current, if not designed for the purpose.
To assure that's not the case, just measure with an Ohmmeter at the (unconnected ring terminals) when the charger is off - it should measure high (several MOhm or more).

Once again, the apparatus is just two ring terminals (that connect to the battery), some wire with a fuse box mounted with a 15Amp fuse, and a plastic wire connector that can be plugged directly to the charger mechanism. So, what I could do is keep those two ring terminals attached to the battery permanently. Whenever I want to charge the battery, I just plug the plastic wire connector into the charger and apply power to the charger. That way I wouldn't have to take off the duct tape and put on the alligator clips every time I want to charge the battery.

In your case, with just a couple of leads with an in-line fuse, no problem, no matter the charger design, as it is disconnected when not charging.

You could cut the cable so that you could mount a suitable connector somewhere as far from the battery as possible, eg. on the back or the front of the 'bot. Then you just plug (the rest of) the cable with  mating connector into the 'bot when you want to charge it.

By the way, the SLA battery only powers the motors through the Sabertooth 2x25...so I would normally have the kill switch off when I'm charging the SLA battery, since I don't want the robot to go anywhere anyway. But if the kill switch is on and power is going through the battery to sabertooth, it'll likely overshadows the 2 amps coming from my charger anyway (with the motors running). So that said, assuming my battery didn't totally explode, the worst I can expect is my robot to just rip itself away from the power socket! 

I would just have to worry about a voltage spike damaging the electronics of the Sabertooth should the robot really pull itself out the socket while power is going into the Sabertooth.

If you are worried about that, you could build this circuit: http://that.homepage.dk/PDF/Charging_Motor_Decoupler.pdf which cuts the connection to the motor, through the normally closed terminals of an automotive relay, whenever it is charging.

So assuming the way you explained things, Soren, the battery shouldn't explode. That's a relief! If the motors are running and its pulling more than 15amp from the charger, the fuse on the apparatus will go out and the battery charger isn't damaged and no fires start. (I also have a fuse on the robot).

If the motors draw more than the charger can deliver, the battery will supply the rest, you will never blow the fuse unless shorting the charger end of the cable while it's mounted to the battery via the ring terminals.
I wonder why they used a 15A fuse if the charger only provides up to 2A max. though.

[omkar]

Hi,

I have a similar problem but I am using a NiMh battery. I have a 9V NiMh rechargeable battery that can be charged within 1.5 hr at 200 mA current. This battery is going to be powering some of the sensors in my robot. These sensors do not like surges of any kind (voltage or current). The robot is programmed to navigate (or be guided) to the charging station at a regular intervals. Can I recharge this battery in any way so that I dont have to turn off the circuits. I was thinking of using a simple PNP diode which will cut off the main circuit from the charging circuit as soon as there is a charging current flowing through. But this causes a problem that I wont be able to get the robot to turn on and move out of the docking station. So.. Can I charge the NiMh battery without having to turn the robot off? Is there any circuit out there which will enable safe connect and disconnect from the charging station preventing the surges which usually occur when contacts come close together? Also what will be the impact of the charging current on my circuit. Currently the circuit works on 8V 250mA current.

Any insight into this is appreciated.
Cheers