Dead battery charging Pop Quiz

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

When a car is jump-started from a dead battery, and the charging system is healthy and putting out 14-14.5V, what is the initial charging current?

I believe it can be calculated knowing the internal resistance of the battery and with knowledge of what the voltage is of a dead battery. Looking for theoretical and/or practical knowledge here.

Main question: Is it under 50, 75, or 100 amps?

George Bright · **Posted:** Mon Feb 07, 2005 9:04 pm

The answer would depend upon a number of factors. The internal resistance of the battery and the resistance of the cables to the battery. Also how many cells have been discharged to 0 volts. Is the battery a normal cranking battery or a deep cycle type? The house batteries on my boss' motor home settle down to a 60 amp charge fairly quickly with the terminal voltage at about 12.5V.

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Jeez, so many questions. I'm just trying to figure out what size relay is NEEDED for a switched accessory battery in the bus. Worried right now about inrush current.

Dunno the internal resistance. What's typical? I have seen internet references to 0.01 to 0.1 ohm.

Wire size? We can choose it. Maybe we WANT something smaller to control inrush current flow.

Deep cycle. Why does that make a difference? Need to know.

Assume none of the cells are 0 volts. Say the battery is at 1/4 charge. I believe that means the voltage is at 11.5 or so volts, right?

Patrick Wellenius · **Posted:** Tue Feb 08, 2005 1:48 am

If you assume the 'dead' battery is at 11.5V (no idea if this is a good assumption) and the 'good' battery at 12V, you'd have a charging current of 50A (using R = 0.01 ohms). Using the lower internal R gives a safer estimate of what the current could potentially reach. If you assume a 14V 'good' battery, you'd see 250A through the relay. Those resistances are critical though. Optima claims as low as 0.003 ohms for the redtops which would put a real hurtin on the system.

You don't want to use the wire to limit your current. Doing so will create a fire hazard. I'm not a power systems guy, but you could likely find a large enough resistor to handle the load that can limit the current to something reasonable (ie, less than what the relay and wiring can take). Don't forget your fuses! For batteries, go with at least 4AWG as the bare minimum (good for ~ 75-80A). Go for 0AWG if it's in the budget (good for ~175-200A).

jimpastorius · **Posted:** Tue Feb 08, 2005 8:52 am

MikeWhitney wrote:

Jeez, so many questions. I'm just trying to figure out what size relay is NEEDED for a switched accessory battery in the bus. Worried right now about inrush current.

What are you trying to do? Do you want to be able to switch between batteries?

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Patrick Wellenius wrote:

If you assume the 'dead' battery is at 11.5V (no idea if this is a good assumption) and the 'good' battery at 12V, you'd have a charging current of 50A (using R = 0.01 ohms). Using the lower internal R gives a safer estimate of what the current could potentially reach. If you assume a 14V 'good' battery, you'd see 250A through the relay. Those resistances are critical though. Optima claims as low as 0.003 ohms for the redtops which would put a real hurtin on the system.

You don't want to use the wire to limit your current. Doing so will create a fire hazard. I'm not a power systems guy, but you could likely find a large enough resistor to handle the load that can limit the current to something reasonable (ie, less than what the relay and wiring can take). Don't forget your fuses! For batteries, go with at least 4AWG as the bare minimum (good for ~ 75-80A). Go for 0AWG if it's in the budget (good for ~175-200A).

OK, then I guess I'm looking for practical experience then.

Let me ask the question again - if you have a dead battery (dead enough that it won't start a car but still "alive") and you jumpstart it, how many amps are going into the battery when the car starts (and jumper cables are removed).

Maybe I should just go outside and measure myself. I thought someone would be able to figure it out.

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

jimpastorius wrote:

MikeWhitney wrote:

Jeez, so many questions. I'm just trying to figure out what size relay is NEEDED for a switched accessory battery in the bus. Worried right now about inrush current.

What are you trying to do? Do you want to be able to switch between batteries?

I deleted my previous offhand comment and associated replies.

Quote:

7. Battery isolator - IN PROGRESS? FOR NEXT BUS DAY

Still kicking around the best place to mount these. Electronics guru Gordon Taylor is working on an auto-sensing circuit to switch the relay OFF if the bus isn't running. Decided NOT to use the RV isolator, since it would require significant re-wiring of the alternator wiring. Still looking for the perfect sized box to mount 2 batteries on the floor.

I was trying to keep this discussion simple since I have one simple piece of information I need to figure out. We're planning to install 2 accessory batteries on a switched circuit that drive all of the club electronics during an event. This will prevent the bus batteries from discharging at all during the event and will ensure that the high starting current needs of the diesel are always met.

Right now I need to know what the worst case current draw will be so (1) we can determine the right size relay to buy, and/or (2) how to limit the current so that it will not exceed the relay capacity.

The situation to think about is this: Worst case during an event, the accesspry batteries will be drained to 25% or so. The accessory batteries will either be 2x Optima Yellow or 2x existing deep cycle batteries.

When the bus is started the auto-switch box will connect the accessory batteries to the bus power system. Think about it this way: This is no different than jumpstarting a dead battery in terms of alternator output and charging current.

This is why I framed the question this way. The reality is that there is resistance in the wire and in the battery, and the alternator has a decreasing voltage curve with current. I bet that a normal jumpstarted car is charging initially with something like 50amps. Just a hunch. Need to find out.

As far as wire sizes and runs, the relay will be tied to the main batteries with an existing wire that looks like 4 or 2 gauge. Wiring between the relay and the 2 batteries is open for discussion. If the current is 50 amps, 8 gauge is probably fine for such a short distance. If it's 100 amps, we have some problems that need to be solved.

Unfortunately, it sounds like the best way to do it is just make some assumptions, hook it up, and keep a fire extinguisher handy. OK, I'm exaggerating. We'll put in a circuit breaker sized to the relay rating.

Patrick Wellenius · **Posted:** Tue Feb 08, 2005 11:41 am

maybe someone in the club has a clamp-style ammeter? find a dead battery, hook it up to a good one and measure the current. remember to wear eye protection

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Patrick Wellenius wrote:

maybe someone in the club has a clamp-style ammeter? find a dead battery, hook it up to a good one and measure the current. remember to wear eye protection

Need to hook up to a running 14v auto to simulate correctly.

Remember, delta V is only going to be 2V, not 12V like when there is a dead short. Significantly less current.

Patrick Wellenius · **Posted:** Tue Feb 08, 2005 12:21 pm

you're right about the test situation.

with such small internal R's, you would still see very large currents. 2v/0.01ohms = 200A!

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Patrick Wellenius wrote:

you're right about the test situation.

with such small internal R's, you would still see very large currents. 2v/0.01ohms = 200A!

It is quite bloody unlikely that the alternator can put out more than 100 amps at idle to these batteries. That means that there is a decreasing voltage curve from the alt with increasing current.

I'm going to stick to my 50-75 amp first guess. Anyone want to test this? Got a dead battery? And a clamp-on ammeter?

Patrick Wellenius · **Posted:** Tue Feb 08, 2005 12:47 pm

Then say the max possible current is the output of the alternator, but some alternators can do well over 100A, even at low rpm.

George Bright · **Posted:** Tue Feb 08, 2005 8:20 pm

If this is for the bus then I would guess that a 75A relay should handle the current needs. The alternator will also be charging the bus starting batteries right after cranking too. I don't know the physical layout of the batteries but I would guess that the aux batteries would have longer leads to the alternator and hence would have a lower inrush current.

Patrick's answer does not take in the resistance of the cables, terminals, the isolator relay, and the return path through the frame.

The short answer is 75A should cover it.

Patrick Wellenius · **Posted:** Tue Feb 08, 2005 9:28 pm

I'm a grad student - I live in a theoretical world. In theory.

DickRasmussen · **Posted:** Tue Feb 08, 2005 10:56 pm

Mike,

If this link works look at the dual battery switches:

http://www.go-rv.com/coast/do/catalog/p ... ageNum=111

Per this RV catalog: A simple mechanical relay used for isolating starting and auxiliary batteries in RV's is rated 80 Amps CONTINUOUS.

If you are concerned that each battery can draw enough to exceed 80 Amps total for more than the first few minutes, you could use a separate relay and wiring for each battery.

I bought one at Napa years ago for our camper van. The paper RV catalog I have has these relays for under $17.00. Napa may be cheaper.

When buying one at an autoparts store, be sure to specify you need the version with a continuous duty coil (i.e. not a starting relay).

Dick

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