“The charger has automatic V(Delta Peak) cutoff and automatic trickle charge”, says the web site. When I put a 6V battery (LM BAT-05) on it overnight, it charged the battery to over 7V. When I put a 7.2V battery (LM BAT-04) on it overnight, the battery ended up at 8.5V. When I measure the charger output with a voltmeter, I see 12V. Both chargers behave identically.
Is this how the charger is supposed to behave?
Is this going to damage my batteries?
I burned out an HS-645 servo last week, and I think it may have been due to using the overcharged 6V battery to power a servo at 6.8V. Is that likely to cause a burnout?
Is it safe to use a USC-01 charger as a power supply, e.g., to supply 12 volts to power some motors through a Sabertooth 2x10 R/C?
Thanks in advance for your help with these questions. We finished building our crab arm rover and I’ll be posting pics on Friday.
The voltage ratings on NiMH cells are a nominal rating. Their voltage will be slightly higher with a fresh charge, and lower as they near depletion. This is a normal condition which is exhibited by nickel metal hydride batteries.
Although I am not familiar with the absolute maximum ratings of the HS-645 servo, I have run them at higher voltages than that for extended periods with no ill effects. Was the servo under load, holding up a weight, or supporting a structure when it failed? Even when sitting still, servos are working hard if they’re resisting motion on their shafts.
I’m not sure about the suitability for using the charger as a power supply - I would think that the peak detection and switching circuitry would make it unsuitable for such a purpose.
Yes. When you charge a battery, a slightly higher voltage is used. When you are charging your 7.2Volt pack, it is probably putting out just over 8.5 volts.
You are reading 12 volts with your meter because having your voltmeter hooked to the charge jacks, instead of a battery, is tricking the charger into thinking it’s not getting any resistance from the pack and it can’t detect the size of the pack. So it is using it’s maximum charge voltage.
No. It is completely normal. A 7.2 Volt battery will sit slightly higher than that. The pack will sit at 7.6 Volts fully charged or even over 8 Volts when fresh off the charger. It will drop to 7.4 immediately after being put under load. And drop below 7 under heavy loads or near 50% discharge.
This is true for all batteries but is especially pronounced in NiMh and NiCd cells.
No and No. 6.8 volts won’t burn out an HS-645 unless it is stalled for extended periods, which will burn out any servo even at lower voltages. And your pack likely was putting out 6 Volts under load. Not 6.8.
No. Some do but I highly discourage it. Chargers aren’t designed to produce stable voltages. The frequently spike and drop. Not noticeably or even measurably on a voltmeter sometimes. But they do jump around. This can be very hard on a Speed controller. A speed controller provides full voltage in intermittent surges. The faster it provides them and the longer the voltage is held, the faster the motor goes. The speed of those flashes is determined by the throttle control. If the input voltage is unstable, it puts undue stress on the controller as it attempts to compensate it’s pulses. The difference may not be enough to damage your H-bridge at these types of low voltage, low amp applications. But it can be devastating in a high amp, high volt combination.
Also, if your charger has auto trickle detection, it senses the resistance of the pack you have connected to it in order to sense when it’s fully charged. Having a speed controller hooked to it could cause your charger to jump in and out of trickle mode. Stressing your charger. The constant variations can also stress your controller.
I would not recommend it.
Thanks to everyone for their informative answers about the charger and batteries.
The HS-645 that burned out was in a Robotzone powered gearbox. It was not under any appreciable load when it failed, and we had used it for less than an hour total time. I swapped it for another HS-645 we had laying around, and that one has run for much longer periods with no problems at all. So I’m guessing the servo was just defective. I’m sending it back to Robotzone and they’re going to take a look at it.
I also had a question on this charger. Theres a switch to select between charging rates. This is the first charger I’ve owned with sucha feature. I am using a bat-03 6v 1600mah pack. I assume charging at 1.2 amp will charge it faster, but are there any cons to this or pros to charging at 600 ma?
The 1600mAh packs should be charged with 600mA, the 2800mAh packs can be charged with 1.2A. If you are in a hurry you can charge the 1600mAh pack with 1.2A but if the pack gets too hot it can cause damage. If it get’s a little warm no problem though.
