0
\$\begingroup\$

I want to experiment with some 120V BLDC motors. I will need hundreds of amps at times. The controllers can typically take a bit more than 120V input, say 136V. I don't want a huge battery bank and complicated (expensive) charger. I'm thinking of ways do something cheap and easy.

Let's say I use 10 "12V" lead-acid batteries in series. Normally they'd be charged to 135V or so. I'll switch a full-wave rectifier with an SCR and connect the output directly to the batteries. If the voltage gets over 130V(?), I'll simply cut the SCR for awhile. This should keep it below "float charge" range of the battery. (I'll also need to limit current in case I pull the battery way down.)

I understand that this is not optimal and it wastes much of the potential of the batteries, but would it be safe? Is there a simple way do better?

\$\endgroup\$
  • \$\begingroup\$ XY problem. Why SCR? How will you limit the current? How will you go without galvanic isolation? \$\endgroup\$ – winny Dec 31 '17 at 23:33
  • \$\begingroup\$ Can I safely use rectified 120VAC to keep a 120V lead acid battery charged? .... Yes, with appropriate charging circuit. \$\endgroup\$ – jsotola Dec 31 '17 at 23:49
  • 1
    \$\begingroup\$ 120 volts RMS is 170 volts peak, so no, not directly. \$\endgroup\$ – τεκ Dec 31 '17 at 23:51
  • \$\begingroup\$ I have charged lead-acid batteries a few times with a just a variable transformer and a rectifier, no isolation. I was careful and kept a close watch on the current and adjusted the voltage accordingly. I don't think it is a good idea to just shut off the charging circuit when the voltage gets to a certain level. \$\endgroup\$ – Charles Cowie Dec 31 '17 at 23:57
  • 3
    \$\begingroup\$ I have safely charged a dead 12V battery in winter with 120Vac , a toaster and a power diode, but without awareness on battery imbalance, and how to prevent it, It is an accident waiting to happen. \$\endgroup\$ – Sunnyskyguy EE75 Jan 1 '18 at 0:08
1
\$\begingroup\$

TL;DR Any quick/cheap method will probably damage and shorten the life of the batteries, especially if connected in a big series string.

A battery charger is more than just shoving charge into a battery. It also has to do no damage, to you, and to the batteries.

To you. It needs to be isolated from the mains. This means a transformer.

To the batteries. That means you have to respect the maximum charge rate and voltage at all times, which means a peak measurement. Unfortunately when you measure raw rectified AC, most meters will make an average measurement, and so underestimate the effect of the current on the battery. If you are measuring it with a meter, you should use smooth DC to get correct measurements.

You don't say whether you are using sealed or wet cells. Wet cells have greater tolerance to overcharging, as you can replace the lost water by topping up, although gassing 10 batteries at the same time will generate a lot of explosive gas, make sure you have adequate ventilation. Sealed cells must not be allowed excessive current during float, as this will result in a permanent loss of water to the battery. Usually this is done by limiting the voltage to around 2.3v per cell (with ideally a temperature coefficient applied), but can also be done by checking the long term float current (expect current around C/1000, C/100 is too high).

Normally, for battery balance, I'd say you don't have to worry about for lead and nickel chemistries, and they balance themselves on over-charge. However, this is only at float currents. With 10 batteries in series, if you're only monitoring the voltage from end to end, one cell could suffer severe water loss while the other cells are still accepting charge. Unfortunately, with the target float current in the C/1000 region, that makes for a very long float charge and balance if done by float overcharge alone.

If you want to recharge your batteries between experiments at a reasonable rate, then you need at least some form of per-battery voltage monitoring, if not active balancing, if you do it with them connected in series.

\$\endgroup\$
  • 2
    \$\begingroup\$ It doesn't necessarily need to be mains isolated - just treat the batteries and everything connected to them as mains potential. Given that it's a 120V battery system, you should be doing this anyway. \$\endgroup\$ – Someone Somewhere Jan 1 '18 at 7:46
  • 1
    \$\begingroup\$ @SomeoneSomewhere yes, you're absolutely right. If you're going to juggle chainsaws, you might as well have them running! \$\endgroup\$ – Neil_UK Jan 1 '18 at 9:28
  • \$\begingroup\$ @Neil_UK, especially since the blade has a centrifugal clutch, so running saws are hotter, and noisier, but no sharper. \$\endgroup\$ – Jasen Jan 2 '18 at 0:05
0
\$\begingroup\$

Rectified 120V AC will be somewhere over 160V, which would be destructive to the batteries, so nothing about this charger will be inherently safe.

Safety then is something you must add to the system ... and in all its ramifications that is not as simple as "cut the SCR for a while". Neil mentions some of the issues and his chainsaw analogy is about right.

Frankly your best bet will be to find a commercial charger that handles the issues for you and lets you focus on your motors and your application, rather than worrying whether that SCR can fail permanently short circuit (yes it can!) and how to reliably protect the batteries against that event and others.

\$\endgroup\$
0
\$\begingroup\$

if you're going this route, you want an inductor in series with the SCR, to both limit the current and to improve efficiency.

your power company probably wants you to balance the draw so that current is drawn on both half cycles, this means you need a bridge rectifier, and that means both ends of the battery are live.

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.