20-500amp programmable, 12volt, dummy load? - Page 1
Forum section: electronics / projects

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 21, 2014, 09:58:37 pm
content: Basically! I'm looking for a way to easily load test batteries, or battery banks, with a settable 20-500amp load...Is this a possibility?Normal high current load testers use carbon pile setups I believe, but, I'd rather something a bit more high tech...at 12volts, I realistically need 0.024ohm across the battery.. of course, as the voltage drops, I'll need to decrease the resistance... 0.018ohm at 9volts..And, I realistically need something that can dissipate 6000watts (for short periods of time) and potentially 2000watts for longer periods...Now... where do I start?Thanks!

author: Christopher
author_rank: Frequent Contributor
timestamp: March 21, 2014, 10:00:54 pm
content: Gate of a high-power fet (IRFP350?) controlled by a opamp sensing the current. "Loads" of these on heatsinks (10A each?) in parallel across the load.

author: dannyf
author_rank: Super Contributor
timestamp: March 21, 2014, 10:11:30 pm
content: QuoteAbsolutely.QuoteGo to your town hall and get a permit to build a power plant in your basement.

author: johansen
author_rank: Super Contributor
timestamp: March 21, 2014, 10:48:32 pm
content: nichrome wire and a few fans.use a few dozen mosfets to turn them on and off.

author: Marco
author_rank: Super Contributor
timestamp: March 21, 2014, 11:07:05 pm
content: Quote from: johansen on March 21, 2014, 10:48:32 pmTurn what on and off?

author: mariush
author_rank: Super Contributor
timestamp: March 21, 2014, 11:14:20 pm
content: or get a bunch of 12v xenon car bulbs ... 20w each or something like that.Or make something like this :http://www.sleepyrobot.com/?p=136and multiply by 50 or so .. one of those can do 3-5A at 12v.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 21, 2014, 11:25:03 pm
content: Quote from: Marco on March 21, 2014, 11:07:05 pmThe 0.018ohm short circuitlolAh, 20, 50, or even 100watt globes probably aren't practical, as I'm looking at a 6000watt load here.

author: FrankBuss
author_rank: Unknown
timestamp: March 21, 2014, 11:34:02 pm
content: For 0.024 ohm you could just use some thick and long wire, and cool it in a bath tube. Even @2000W it would need some time to increase the water temperature. But the contact resistance to the battery might be a problem. When the voltage drops to 9 V, short-cut some length of the wire to get 0.018 ohm

author: Psi
author_rank: Super Contributor
timestamp: March 22, 2014, 12:48:01 am
content: 6kW 12-230V inverter and a few 2kW bar heaters with triac temp controls.Or pretty much anything that runs from 230V and has a non-thermostat type control for adjusting its power output.The hard bit maybe finding a 6kw inverter that can take 12V, a lot of the high power ones need 24V but im pretty sure ive seen 12V versions they're just not as common.

author: johansen
author_rank: Super Contributor
timestamp: March 22, 2014, 12:49:59 am
content: turn what on and off?you do know what nichrome wire is right?so anyhow, i built an electric kiln from hard/heavy firebrick, i think i used 2/3rds pound of nichrome.i bought 17 gauge iirc, and it sucked 37 amps at 240vac out of the outlet.it just barely got hot enough to melt copper, was basically a 7000 watt heatsink, the 11 bricks arranged in a radial pattern just about got red hot on the outside (i had the whole thing buried in perlite)anyhow, point being you don't need a lot of nichrome.but drawing 500 amps you're going to heat up the terminals of whatever method you use to hold the nichrome very quickly.so at a minimum you're going to need a fan to cool the wires down, even if you let the nichrome radiate the heat away at 1100C

author: Marco
author_rank: Super Contributor
timestamp: March 22, 2014, 12:57:28 am
content: Quote from: AmmoJammo on March 21, 2014, 11:25:03 pmBut that's a really nasty way to modulate the current draw, how useful is a programmable load which you can program to 20 ampere average current .... only with 0/600 ampere instantaneous currents?

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 22, 2014, 01:00:49 am
content: I want something adjustable... and "simple" to setup once assembledI have 7.5kw (total) worth of inverters but they have 10volt shut down, and I probably need lower for my testing application

author: Psi
author_rank: Super Contributor
timestamp: March 22, 2014, 01:06:08 am
content: Shouldn't be that hard to find the undervolt protection and tweak the cutoff.Wont be very efficient under 10V but you don't need that

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 22, 2014, 01:13:43 am
content: Maybe I'll just buy one of these and not worry about longer load testinghttp://www.ebay.com.au/itm/PROJECTA-BLT300-6V-12V-24V-1000AMP-CARBON-PILE-LOAD-TESTER-WORKSHOP-HEAVY-DUTY-/281203600126?_trksid=p2054897.l4275

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 22, 2014, 01:39:28 am
content: Way too expensive, everything.Dumb it down.Nichrome, yeah, sure. But can you find it? Stainless steel, almost as good, much easier to find (probably cheaper too).So...Get a stainless plate, calculate how much is required for each resistance value (thickness, width, length). Assemble your battery-under-test with the bolt-on lugs, beefy copper busbars on the lugs, and use Vice Grips to hold the stainless steel sheets onto the busbars. Probably, bolt down one side, then leave the loose end sprung up so it doesn't normally make a connection but you can still get the clamp on it.Unless you require a stable load, actual hands-free remote or automatic control and so on, in which case, the cost of a proper engineered solution skyrockets with each option.Note that the battery's internal resistance (assuming something comparable to a car battery) is on the order of 10 miliohms, so 1. you'll probably be getting more like 6 or 8V into the load (the battery will warm up!), and 2. if you are going to switch it with transistors, you need way less than 10 miliohms to handle the whole thing (plus the energy stored in nearby inductances; a rig like this might be a few microhenries, which will store a whopping 0.25J of magnetic energy at 500A -- quite a spark if it lets go, and just imagine what that would do to a transistor).Tim

author: johansen
author_rank: Super Contributor
timestamp: March 22, 2014, 01:53:23 am
content: .25J?yeah, that will leave a mark.btw:lead acid battery short circuit current is 30-40 times its amp hour rating for almost all AGM cells.i suspect car batteries are about the same.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 23, 2014, 10:36:41 am
content: Fault current for AGM batteries is 10-20 times the Ah rating as far as I know.I wonder if you have more up-to-date info than me. Quite possible. Please let me know.Dick

author: fcb
author_rank: Super Contributor
timestamp: March 23, 2014, 12:06:30 pm
content: Heat conduction of stainless steel is about 10x less than aluminium - be careful.Nichrome wire is readily available in the UK.

author: richard.cs
author_rank: Super Contributor
timestamp: March 23, 2014, 10:22:07 pm
content: Water resistor?Two stainless plates in a solution of NaOH, lower them in for more current and raise for less. Your battery voltage (and the water conductivity as it heats) will change slowly so how about a simple servomotor that controls the plate height for constant current. You'll produce hydrogen* and the water will boil after a while (minutes) but it's simple, cheap and has the potential to be quite accurate.*So use it in a well ventilated area or vent outside (Or at these currents burn off the H2 at the plates ignitingbeforeany amount builds up).

author: electrodacus
author_rank: Super Contributor
timestamp: March 24, 2014, 12:27:27 am
content: Someone mentioned already but a power inverter 12V to 120 or 220v is probably the best way then attach something that can be controlled and can dissipate that amount of power.By using an inverter you reduce the current that needs to be controlled.That Is what I used when I tested the capacity of my batteries and the internal resistance.Of course 2000 to 6000W will need to be dumped in water or outside your house. Or do the tests in the winter and heat your house

author: johansen
author_rank: Super Contributor
timestamp: March 24, 2014, 01:31:10 am
content: Richard,I seem to recall discharging a 7.2 AH cell through 10 or more feet of 14 awg wire and reading 120 amps.the battery wasn't in good shape either. dead short would have been 33% more, so that's 160 amps or 22Cwhere i got the 30-40 figure is from the manufactuer datasheetsthere is very little variation, and i specifically looked it up to see if there is truly a difference between the different "brands" (deep cycle vs extended standby vs gold plated unicorn..$%*^)--there isn't.it is also difficult to truly short out a battery.the brass terminals on 5 and 7 ah cells will melt and/or glow red.the lead posts on larger cells will melt if you don't have a solid bolted connection at 40C discharge currents.

author: ConKbot
author_rank: Super Contributor
timestamp: March 24, 2014, 10:59:16 am
content: How many battery banks do you have? Can you use additional battery banks as an energy dump? I.e. have a battery bank sitting at ~12.0V open circuit and use that as your energy-sink to absorb the energy from the bank under test? That way your variable resistance is only controlling how much current flows, rather than having to dissipate it all?How repeatable/portable/reliable etc does this need to be? While not the cheapest, 3-4 cheap and nasty chinese square wave inverters, and 3-4 $20 1500W desk heaters will work, be decently adjustable (few hundred watt increments), and fairly portable. You could get it on frame that you could move with a hand truck.Also, how big is the bank? And whats it used for? Just wondering if a large bank of 12v batteries is actually the best solution, AFAIK, series bank of batteries have less problems then a bank of series/parallel batteries, but that is not be practical for every application.

author: max_torque
author_rank: Super Contributor
timestamp: March 24, 2014, 04:53:17 pm
content: I had to do a lot of high current load testing of a low voltage inverter and power system a couple of years ago. After looking into all the complicated and clever ways of doing it, we ended up buying a long coil of copper tubing (8mm od), making a simple wooden jig (about 2m long) to hold this coil of tube "open" (so no turn to turn shorts). We then used a hose pipe to just run cold tap water through the tube (total loss system). Using a milliohm meter we then "calibrated" various taps off this coil (but you can use trial and error and a current meter etc), and hence varried the load resistance as required by just sliding these taps up and down the coil.With a supply of cold water, that rig would completely flatten a large truck sized LA battery, and pull 1000A, without getting excessively hot. If you want to add some inductance to the coils (for inverter testing) then sliding some iron bar stock up the middle adds significant (and tunable) inductance easily!

author: woodchips
author_rank: Frequent Contributor
timestamp: March 24, 2014, 07:06:15 pm
content: Use a water bath. Just an ordinary plastic tub with two large plates at each end, pour the water in, mix some salt to make it conductive and then to vary the current either move the plates closer together or increase the depth of water or increase the salinity. No problems with heat dissipation, the water will absorb lots.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2014, 10:21:38 pm
content: Eww... don't use salt: either the anode corrodes away rapidly (this is a good way to electrolytically corrode unreactive metals -- copper for instance to obtain orange powdered cuprous oxide), or (using a graphite, lead dioxide, platinum or other inert anode) chlorine gas and, eventually, the unstable sodium chlorate.Stainless steel in sodium carbonate or hydroxide seems to be okay though, at least for a few volts.Tim

author: oldway
author_rank: Super Contributor
timestamp: March 25, 2014, 08:53:12 am
content: I started studying a project that would suit for this application .It is an electronic load combining resistors with a MOSFET dynamic load, so that 2/3 or more of the power is dissipated in the resistors , and the load is adjustable in a continuous manner without variation of levels .The resistors are very low cost and made of stainless steel wire (in my case , welding wire 0.8 mm in diameter - acceptable current : 5A)For your project, load resistance should be chosen for 30A/resistor and switching would be done by automotive- relay of 30A.The amount of relays and resistors depends on the total current value.An Hall sensor is used for current measurement and regulation.In parallel , there must be a MOSFET dynamic load 12V 50A 600W which will be used to fine tuning .The assembly is cooled by forced ventilation with a fan recovered from an electric heater.The stainless steel wire is very cheap compared to the Kanthal wire, but its resistance varies with temperature .But that does not matter because it will be offset by fine tuning of the MOSFET dynamic load.

author: TerraHertz
author_rank: Super Contributor
timestamp: March 25, 2014, 09:45:00 am
content: Quote from: AmmoJammo on March 21, 2014, 09:58:37 pmWait, are you talking about batterybanks? Isn't it rare (ie never) for many lead-acid batteries to be connected in parallel? I thought this isn't done because the batteries can't be balanced, and also what happens if one goes short circuit.Battery 'banks' are normally wired in series. Certainly the large banks used for large UPSs (that back up whole buildings) are wired in series, with total output voltage in the 200 to 300VDC region.So why are you talking about needing to draw load from a 12 volt source?

author: oldway
author_rank: Super Contributor
timestamp: March 25, 2014, 10:09:02 am
content: We always hope that topic starter knows what he is looking for.But battery discharge current is Cah/10 or Cah/5.500A discharge current is thus for a battery of 2500Ah or more.Such big batteries are used for 48V telecom applications, but I never seen such a 12V battery....Always only 2V cells, never 12V !

author: ConKbot
author_rank: Super Contributor
timestamp: March 25, 2014, 12:05:10 pm
content: Quote from: TerraHertz on March 25, 2014, 09:45:00 amIf youre running flooded batteries (or AGM with a recombination catalyst) that you can do a high voltage (~15+V ?) balance charge regularly, you can keep all the batteries balanced, but your cells better be identical, have the same age and cycle count.Paralleling lead acid batteries is asking for one to fail eventually. Ive seen a vehicle with a bunch of radios and stuff that had an optima yellow top (AGM) added as a deep cycle for the radios. But a standard flooded main battery. In not to long one of them failed (I think the optima, not 100% sure) because they charge at different voltages.I was doing a ~300AH battery backup for a sump pump and went with 2 6V FLA cells just because it would be more reliable in the long run than 2 paralleled cells.

author: G7PSK
author_rank: Super Contributor
timestamp: March 25, 2014, 10:06:12 pm
content: Get a starter motor and a disc brake drive the brake with the motor the more pressure on the brake the higher the current drawn. A starter motor will easily draw 500 plus amps, use one from a large diesel engine, truck tractor etc.

author: mzzj
author_rank: Super Contributor
timestamp: March 26, 2014, 08:11:21 am
content: Quote from: G7PSK on March 25, 2014, 10:06:12 pmWhy not something steam powered

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: April 19, 2014, 09:26:59 pm
content: This was intended for load testing car batteries, or banks of them in parallel...Was basically wanting to load test batteries like in this video:OR, load them with a lower load, for an extended period, to monitor the discharge characteristics.The banks of batteries would consist of 4 or more 100+ah batteries in parallelI'll be leaving this for now (too many things on the go) but thanks for the ideas.

author: turbo!
author_rank: Regular Contributor
timestamp: April 20, 2014, 09:31:58 am
content: Use a carbon pile with the adjustment stem driven by a servo.Mod the input lead to four wire so you're monitoring at the sense wire.Monitor the amps and volts. How you coordinate the amps/volts/servo is in microcontroller based setup.

author: hibone
author_rank: Regular Contributor
timestamp: April 20, 2014, 07:01:01 pm
content: good luckhttp://www.chirio.com/electronic_load.htm