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Old 20-10-2012, 20:14   #16
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Re: Watch those amps champs!

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Originally Posted by CarlF View Post
I have a couple of old beach towels that I throw over everything and then peel just enough back to get at the spots I need. The towel also makes a good non-conductive place to rest tools.

Carl
That may work for 12 volt single bats, but it is still a huge risk. First a beach towel made of cotton or some other fabric, isn't really an insulator.

Any moisture at all will conduct enough to cause a spark at the very least...now if the bats are at or recently were at gassing stage, then the spark causes an explosion, and the towel adds fuel to the fire. It may even be that the towel creates the environment for the explosion by containing the hydrogen gas cloud in a density that can explode. Now if we are talking 24,32,36, or 48 volt systems then a towel is of no value.

Boiling battery acid, is explosive at 4% by volume in open air. Battery acid starts boiling 12.3 volts at a 12 volt nominal, or 2.1 volts times the number of cells in the bat.

Before working on a live bank, make sure there has been no charge\discharge source to the bats for at least 2 hrs and the area is well ventilated for at least 2 hrs after charge\discharge.

Always disconnect the the ships neg to battery first, then disconnect each neg in the string, secure the cables from arc with a short piece of rubber hose, then secure each neg bat post, with a cap.

Now you can follow the same routine with the pos. cables/posts.

one marine average 12 volt battery has a potential of 1.3kw arc.


Lloyd
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Old 21-10-2012, 07:13   #17
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Re: Watch those amps champs!

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Don't feel bad, it has happened to everyone who has spent enough time doing electrical work.
Dave:

Well, I don't feel bad because at least nothing blew up or caught fire. Just wanted to share what could happen when a wire goes someplace it should not. I've since replaced the lug and am now on a fall cruise.
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Old 21-10-2012, 07:17   #18
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Re: Watch those amps champs!

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I like to slide a short (±4") piece of clear hose over the positive cable lug(s); so that it normally sits over the cable insulation, but can be pulled down to protect the lug, when removed from the terminal post. This helps prevent a “loose” cable from shorting out on a negative post, or ground.
Good idea. I do have some Blue Sea battery covers over the battery terminals normally but, this could be additional protection when working on the interconnects.
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Old 21-10-2012, 07:19   #19
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Re: Watch those amps champs!

Greetings and welcome aboard the CF, Richieg1g.
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Old 21-10-2012, 07:24   #20
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Re: Watch those amps champs!

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I second the safety glasses suggestion. I've been fairly consistent with wearing them and happened to have a pair on when I dropped an un-insulated wrench across the positive terminal and ground point. Three 245 AH six-volt batteries blew their tops off. I was pretty well splashed in acid but not blinded. My clothes fell apart and I had to buy four new batteries. Now I'm very consistent wearing them!
Yikes! I have insulated the wrenches I use for battery work with electrical tape to help prevent shorts from happening or at least minimize the chance. But, glasses are a good idea too especially around flooded batteries.
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Old 21-10-2012, 07:58   #21
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Re: Watch those amps champs!

Another hazard with DC is welding of the two components that come in contact with each other. With AC power there is a zero-volt crossing 120 times a second which makes the arc self-extinguishing. Not so with DC. I had this happen to me with the wires on a big solar panel once. That's a much less hazardous situation than with batteries, but was alarming none the less.

As for protection, I've played around with a couple of things to provide protection, mostly from dropped wrenches.

- For wire end protection, I've cut the fingers off an old glove. You can then slip them over the wire end to protect against unintended contact.

- The other is stolen straight from surgeons in operating rooms, where you cover the person up, but have a hole in the cover just where you are working. A sheet of rubber or neoprene with a hole cut in it works well. I've also used small sheets of plywood laid across the tops of the batteries to protect everything except the spot I'm working on.
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Old 21-10-2012, 10:31   #22
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Re: Watch those amps champs!

Another simple way to help reduce accidents, is to have a dedicated set of bat tools, that are insulated.

Either commercial made, or self-made.

Heat shrink, a ratchet handle all the way, wrenches w/heat shrink with only one working end.

lloyd
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Old 21-10-2012, 10:50   #23
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Re: Watch those amps champs!

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Originally Posted by FlyingCloud1937 View Post
Another simple way to help reduce accidents, is to have a dedicated set of bat tools, that are insulated.

Either commercial made, or self-made.

Heat shrink, a ratchet handle all the way, wrenches w/heat shrink with only one working end.

lloyd
Forgot to add use a different color of heat shrink for each wrench, then at a glance you pick the right tool.

lloyd
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Old 21-10-2012, 11:07   #24
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Re: Watch those amps champs!

Good posts all.

All of the above goes double if you've got lithium batteries.
While installing my 200 aH cells in a tight battery space, I accidentally crossed ONE cell (only 3.4 volts) and got the same result as the OP because the internal resistance of LiFePo4 batteries is a lot lower than LA's.
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Old 21-10-2012, 11:13   #25
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Re: Watch those amps champs!

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Originally Posted by mbianka View Post
Dave:

Well, I don't feel bad because at least nothing blew up or caught fire. Just wanted to share what could happen when a wire goes someplace it should not. I've since replaced the lug and am now on a fall cruise.
No over-current protection in that circuit? You might consider some fusing...

Keep in mind that the short circuit current available from even a group 27 deep cycle battery can exceed 1500A at 70F.. So a 200Ah bank or a battery with a CCA of 700 at 0F will pump a LOT more than 700A into a dead short at normal boating temps. Add a few batteries in parallel and the short circuit capability is then multiplied. It is not unusual to exceed 5000A of short circuit capability in a battery bank.

Some batteries throw a lot more than others into a short. For example the Odyssey TPPL AGM in group 31 size can supply 5000A of short circuit current per battery. A bank of four would be 20,000A of current into a dead short.... Ouch!!!!
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Old 21-10-2012, 11:35   #26
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Re: Watch those amps champs!

Fuse or fuze!

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Old 22-10-2012, 10:25   #27
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Re: Watch those amps champs!

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Originally Posted by Maine Sail View Post
No over-current protection in that circuit? You might consider some fusing...

Keep in mind that the short circuit current available from even a group 27 deep cycle battery can exceed 1500A at 70F.. So a 200Ah bank or a battery with a CCA of 700 at 0F will pump a LOT more than 700A into a dead short at normal boating temps. Add a few batteries in parallel and the short circuit capability is then multiplied. It is not unusual to exceed 5000A of short circuit capability in a battery bank.

Some batteries throw a lot more than others into a short. For example the Odyssey TPPL AGM in group 31 size can supply 5000A of short circuit current per battery. A bank of four would be 20,000A of current into a dead short.... Ouch!!!!
Well the lug did act as a fuse in this case. The bank is fused on the output which was disconnected at the time. Putting fuses in the battery interconnects would still require 100 amp fuses and add more connections and possible failure points into the circuitry. I do have the new helm battery monitor wiring fused at the battery connection ends though. I will certainly be more cautious should I have to do something with these connections in the future.
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Old 22-10-2012, 10:30   #28
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Re: Watch those amps champs!

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Originally Posted by twistedtree View Post
Another hazard with DC is welding of the two components that come in contact with each other. With AC power there is a zero-volt crossing 120 times a second which makes the arc self-extinguishing. Not so with DC. I had this happen to me with the wires on a big solar panel once. That's a much less hazardous situation than with batteries, but was alarming none the less.

As for protection, I've played around with a couple of things to provide protection, mostly from dropped wrenches.

- For wire end protection, I've cut the fingers off an old glove. You can then slip them over the wire end to protect against unintended contact.

- The other is stolen straight from surgeons in operating rooms, where you cover the person up, but have a hole in the cover just where you are working. A sheet of rubber or neoprene with a hole cut in it works well. I've also used small sheets of plywood laid across the tops of the batteries to protect everything except the spot I'm working on.
Like the glove idea. I'm got a pair of leather gloves used when handling the anchor that have seen better days might be time to reuse them for something else like you suggest.
The surgeon apron is also a good idea. I usually put rubber mats around the terminals to prevent any of the nuts or washers from falling down between the batteries.
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