Will my car battery explode? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 04, 2018, 11:23:34 pm
content: Hi allI am totally new to electronic, but a "non disclosure" person on a certain YouTube channel, have awoken me to the fantastic world of electronic! :-)My nickname is quiet feting, I am like a mule, looking down on some obscure gizmos with totally fried hair, not understanding what just happened:-)Now to my question:I am starting from scratch and want to try to use my old car battery and an old car 12v relay.The relay have an internal resistance of 160m, would that make a short circuit?If so, would it be smart to add a resistor?

author: james_s
author_rank: Super Contributor
timestamp: February 04, 2018, 11:25:58 pm
content: The relay coil is 160mOhm? That sounds awfully low.It's pretty hard to make a car battery explode, but it's quite easy to burn up wires. My suggestion is to put an inline fuse right off the battery, sized appropriately for the wire you're using. Then if you short circuit something the fuse will blow.

author: Nusa
author_rank: Super Contributor
timestamp: February 04, 2018, 11:33:42 pm
content: Look up Ohm's Law. Understand it, memorize it. You'll need it constantly.In this case, the current through the relay coil will be I = V/R = 12 Volts / 0.160 Ohms = 75 Amps. Quite a lot, but not a short circuit.Although as James says, that's a suspicious value for the relay. Are you sure it isn't 160 Ohms? In which case it's 12 Volts / 160 Ohms = 0.075 Amps.edit-fixed my mega vs milli brain-fart

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 04, 2018, 11:35:17 pm
content: There are relays that have very low impedance coils, for sensing current in applications such as detecting burned out light bulbs. (That's more commonly done with opamp circuits detecting voltage drop nowadays.)BTW, one super cheap and easy way to make such relays (very uncommon in small quantities) is to coil some wire around a drill bit or similar and then insert a reed switch in the coil.

author: Zero999
author_rank: Super Contributor
timestamp: February 04, 2018, 11:39:45 pm
content: Quote from: Nusa on February 04, 2018, 11:33:42 pmI've never seen a relay with a coil with such a high resistance as 160M Ohms. A lower case m suffix means milli, which is *0.001, so 160mOhm is 0.16Ohm, which will cause a large current to flow.My guess is it's a typographical error: the correct relay coil resistance is 160Ohms, but 160m is a possibly if it's a current sense relay, as mentioned above.

author: glarsson
author_rank: Frequent Contributor
timestamp: February 04, 2018, 11:41:29 pm
content: The data for the relay probably specified the internal contact resistance as 160m\$\Omega\$. The coil will have a more reasonable impedance.

author: tautech
author_rank: Super Contributor
timestamp: February 04, 2018, 11:43:05 pm
content: Quote from: FriedMule on February 04, 2018, 11:23:34 pmYou looked at the datasheet wrong, 160 mwill be contact resistance.....you need coil resistance and it'll be in the 100-2000range.Be sure the coil is DC rated and no, there's no need to add a series resistor.

author: IanB
author_rank: Super Contributor
timestamp: February 04, 2018, 11:53:44 pm
content: As someone mentioned above it's very dangerous to work with a car battery unless you put a fuse between the battery and your circuit. A car battery can supply hundreds of amps, which means any accidental short circuit can quickly result in white hot molten wires, smoke, fire and burns if you are touching the thing that goes white hot. It's no fun to be holding a wire that suddenly goes red hot under your fingers.Use an automotive fuse of about 10 A rating in an inline fuse holder and make sure everything you connect is after the fuse. It might also be sensible to wire in a switch so you can turn the power off quickly in case something does go wrong.

author: james_s
author_rank: Super Contributor
timestamp: February 05, 2018, 12:02:23 am
content: Quote from: NiHaoMike on February 04, 2018, 11:35:17 pmThe bulb failure sensor in my car is ingenious, it uses a set of reed switches, each surrounded by a coil made of two parallel wires. These coils are in series with the left and right lamp of a given location but are wired out of phase. This means that with both lamps functioning, two equal but opposite magnetic fields are created which cancel each other out. If one bulb fails then you get a magnetic field which closes the associated reed switch, illuminating the bulb failure indicator.Of course if both lamps fail then the failure warning doesn't work. It also tends to be a bit too sensitive so a bit of corrosion on a bulb socket or connector, or even different brand bulbs can cause false bulb failure warnings.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 05, 2018, 01:39:39 am
content: WOW!!You are so fantastic, I am not sure about my measurement, own a cheap 15 dollars multimeter, want to invest later:-)Thanks a lot for the great advice about the fuse, I'll defiantly add a fuse, low amp! :-)I found some data, hope they help:Contact Capacity:• Normal switching Capacity (res.load):- NO 40A 14VDC- NC 30A 14VDC• Max. switching current: 40A• Max. switching voltage: 30VDC• Max. switching power: 560WCharacteristic Data:• Initial contact resistance (at 6VDC 1A): 50m? Max.• Operate time (at nominal volt.): 10 msec. Max.• Release time (at nominal volt.): 5 msec Max.• Initial insulation resistance: 100M? Min. (DC500V)• Initial dielectric strength:- Between open contacts: AC500V, 50/60 Hz 1Min.- Between coil and contact: AC500V, 50/60 Hz 1Min.• Vibration resistance:- Functional: 10 ~ 55Hz at double amplitude of 1.5 mm- Destructive: 10 ~ 55Hz at double amplitude of 1.5 mm• Shock resistance:- Functional: 20G Min.- Destructive: 100G Min.• Endurance (operations):- Mechanical (at 7,200 ops./h): 10,000,000- Electrical (at 600 ops./h): 100,000• Ambient Temperature: -40°C ~ +125°C (no condensation)

author: IanB
author_rank: Super Contributor
timestamp: February 05, 2018, 02:43:52 am
content: Unfortunately none of that data is about the energizing coil. There should be some data about the nominal coil voltage, the closing current/voltage, the holding current/voltage, and other related information?

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 05, 2018, 03:26:06 am
content: Hmm I found a info, that may be what you are asking for:coil current 175 mA at 13.8V DC

author: Old Printer
author_rank: Frequent Contributor
timestamp: February 05, 2018, 04:48:31 am
content: Your car battery, or any lead acid battery can explode, under the right circumstances. I know this from first hand experience, and might be blind except for wearing large reading glasses and a bit of luck. A LA battery under charge gives off hydrogen gas. Under a very high rate of charge, a lot of hydrogen gas, which is very explosive if exposed to a spark. Between that and the stored potential energy they deserve a lot of respect, and will also eat your shirt and pants with slight contact.

author: Nusa
author_rank: Super Contributor
timestamp: February 05, 2018, 05:59:30 am
content: Quote from: FriedMule on February 05, 2018, 03:26:06 am13.8V is about the right voltage for a 12V vehicle when the engine is running.So if you measure your coils resistance, you should get about R = V/I = 13.8/0.175 = 79 ohms if your info is correct.

author: james_s
author_rank: Super Contributor
timestamp: February 05, 2018, 07:21:46 am
content: Note that many meters are not very accurate below a few Ohms, but that shouldn't be an issue for a relay coil. Are you sure you're measuring the correct pins?

author: flolic
author_rank: Frequent Contributor
timestamp: February 05, 2018, 08:11:30 am
content: Quote from: Old Printer on February 05, 2018, 04:48:31 amTwo car batteries connected in series, used for a old online UPS. One morning loud explosion wake me up, that was in a room next to my bedroom.That UPS pushed a little too high charging voltage that caused batteries to loose water rather quickly. I knew about that problem, but ignored it...That room was my storage room full of various electronic crap that all get covered in sulfuric acid, real mess.Be careful with batteries, of any kind...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2018, 08:22:38 am
content: FWIW, the biggest relay / contactor / solenoid in a car is the starter, but that shouldn't draw more than a few amps (several ohms resistance).Tim

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 05, 2018, 10:02:52 am
content: Thanks for the warning! It looks like it is a fairly dangerous project! I'll be wary carful!You are all so great, this forum are wary welcoming and inviting!!If it a 79 ohm coil in the relay is the only resistance on a 12V car battery, what would you advise me to`1) Extra resistor (what value / what about voltage drop?).2) Try and run? :-)

author: tautech
author_rank: Super Contributor
timestamp: February 05, 2018, 10:22:41 am
content: Quote from: FriedMule on February 05, 2018, 10:02:52 amNO coil resistor needed.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 05, 2018, 07:25:21 pm
content: Sorry for me being confused, but some say that it will explode or burn and other say that it will work just fine! Or am I not reading the answers correct?

author: Gyro
author_rank: Super Contributor
timestamp: February 05, 2018, 07:54:08 pm
content: I think the general consensus is that there is no way your relay coil is 160m, it is rated at 175mA 13.8V (approx 80). It is safe to use the coil at battery voltage without any resistor being needed.The Safety aspect is that you MUST have a series fuse as close as possible to one battery terminal to stop it burning all your wiring if you create a short (a car battery can source several hundred Amps into a short circuit).If you directly short the battery with something like a heavy spanner, then yes, it might explode. Certainly the spanner will light up!The battery can also explode if you charge it at too high a rate and voltage, not a problem if it is still connected to your car or used with a proper battery charger.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 05, 2018, 08:14:36 pm
content: Gyro thanks a lot for your reply, it made it all clear for me!!Thanks to your all for helping, I am wary surprised and happy for how you reacted to a newbie as me!!My conclusion are that a car battery are maybe not are the best to start with but it is all I have at the moment.I'll put a low amp fuse direct on the + lead connector and make two really long 18 awg cable, so they burn before me:-)And I am trying to remember and understand the most important formula's P=I/V and V=I/R (or P=I/U and U=I/R as it is written in Denmark).

author: james_s
author_rank: Super Contributor
timestamp: February 05, 2018, 08:48:15 pm
content: I've never seen a lead acid battery explode just from overcharging, you can certainly boil out the water and damage it though, that happened once to a friend's car when the voltage regulator failed. As has been mentioned above, a direct short can cause them to rupture though as the tremendously high current a car battery can produce is able to rapidly boil the water inside. As long as you use some common sense though they are fairly safe as far as large capacity batteries go. I do recommend putting it in plastic bin as there is almost always some acidic vapor. Also take care when charging as a small amount of hydrogen is produced, make sure it can escape and float upward. As has already been covered, put an inline fuse in the circuit connected directly to the battery, sized appropriately for the wire you are using. A smaller fuse is fine too but don't go bigger than the wire is rated, there are ampacity charts out there if you need a reference.Just be careful and you will be fine. Stored energy is inherently dangerous but a little care goes a long way toward making it safe.

author: FriedMule
author_rank: Frequent Contributor
timestamp: February 05, 2018, 09:20:04 pm
content: james_s thanks for your great advice! and comments:-)I'm thinking about, buying a cheap adjustable power supply later on, so that I can get rid of that battery and be a lot safer :-)

author: Nusa
author_rank: Super Contributor
timestamp: February 05, 2018, 09:26:52 pm
content: Quote from: FriedMule on February 05, 2018, 08:14:36 pmIt's P = I * V, known as Watt's Law. Which of course can also be stated as I = P / V or V = P / I. P being power in Watts.Ohm's law (V = I * R) and Watt's law are often combined and restated to solve for an unknown if you know any two of the other values.Feel free to use U instead of V, you'll be understood. Some references use E instead of V.edit -- I goofed too! Fixed.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: February 05, 2018, 09:57:38 pm
content: Quote from: Nusa on February 05, 2018, 09:26:52 pmNusa, I know you meant to type the right formula for Watts, but those fingers sometimes...P=I*V, I=P/V, V=P/INot trying to be pedantic, I typo all the time too.EDIT: Fixed typo where '-' changed to '='

author: IanB
author_rank: Super Contributor
timestamp: February 05, 2018, 10:05:37 pm
content: Quote from: FriedMule on February 05, 2018, 07:25:21 pmQuote from: james_s on February 05, 2018, 08:48:15 pmThe danger is this:When you charge a lead acid battery it produces hydrogen gas. Hydrogen gas will be vented from an unsealed battery, and possibly from a sealed one too. Hydrogen gas forms a very explosive mixture with air. Therefore, hydrogen + spark = explosion.This is why the instructions for lead acid batteries always warn to use them outside or in a well ventilated area. Do not use such a battery in an enclosed space, and especially do not charge the battery in an enclosed space.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2018, 05:31:10 am
content: FWIW, hydrogen dissipates quite quickly; you'd have to have a room so small, so tightly enclosed, as to border on "intentionally concentrating vapors", to get it to blow up a room.That said, hydrogen's explosive limit is quite wide, and blowing up a room would be quite a problem indeed. Worth being aware of, and easily avoided. (The most obvious exception I can think of, is anything with a solid structure, like the plate-welded rooms of a ship. In such conditions, asphyxiation from welding gas is a very real problem.)The enclosure of the battery itself does serve as confinement, though, so if a source of ignition occurs within the battery, you might get something like what's pictured above!If you have the time to spare, a slower charge will produce less hydrogen (and cause less grid corrosion, and more desulfation action, plus allowing even more time for hydrogen to dissipate). A fixed voltage float charge will naturally cease charging, once all the active area is fully charged: it takes more potential to dissociate water, so staying below that threshold, you can only drive the battery towards a full charge. It might take 24+ hours to reach this state, though. The voltage needs to be fairly precise as well (a rectifier charger won't do, but a switching supply can do, no problem), and adjusted for the particular battery chemistry and temperature (so, if we're talking about flooded lead acid at room temperature, is pretty well defined).Tim