# Help! What does my starter motor draw?



## staceyneil (Nov 4, 2000)

Redoing the electrical system and I need to figure out the amperage draw for the starter motor in order to size the battery, cable, slow-blow fuse, switch, etc. It''s a late-model Perkins 4-108. Any help appreciated!!
Thanks
Stacey
http://www.sailnamaste.com


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## TSOJOURNER (Dec 16, 1999)

Hi Stacy,
I''ve been holding off answering your questions hoping someone with more expertise would provide the information. 
Since you haven''t received any info yet, I will attempt to answer some of your questions.
Based upon automotive starters I would say your starter will draw over 200 amps... maybe as much as 350...because of this high current flow, it probably isn''t practical to try to fuse the circuit.

From a practical standpoint, should the starter fail, it likely will be when you are using it. Worst case scenario is the starter is shorted to ground and the starter cable is smoking... maybe on fire. Since you are using the starter and it isn''t responding you will go below to see what''s the problem. Smoke and fire should provide a pretty good clue! 
-A good system will have a battery switch to isolate the battery from the short...throw the switch and the problem is solved. (maybe you need to use the fire bottle to put out the fire, but the boat is not likely to go up in flames). So a Battery switch will normally provide reasonable protection from a short...forget the big fuse. 
When away from the boat, the battery switch in the ''off'' position will isolate the battery from potential system shorts.

If you plan to use a dedicated starting battery, one with a 1000 amp cranking capacity will do a good job. I have one on my boat and it turns my 4-108 over with much more authority than the deep cycle house batteries I used before.

I don''t consider myself an expert on this but there''s my two cents worth.

Fairwinds,

Jim


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## TSOJOURNER (Dec 16, 1999)

Hi Stacy,
I''ve been holding off answering your questions hoping someone with more expertise would provide the information. 
Since you haven''t received any info yet, I will attempt to answer some of your questions.
Based upon automotive starters I would say your starter will draw over 200 amps... maybe as much as 350...because of this high current flow, it probably isn''t practical to try to fuse the circuit.

From a practical standpoint, should the starter fail, it likely will be when you are using it. Worst case scenario is the starter is shorted to ground and the starter cable is smoking... maybe on fire. Since you are using the starter and it isn''t responding you will go below to see what''s the problem. Smoke and fire should provide a pretty good clue! 
-A good system will have a battery switch to isolate the battery from the short...throw the switch and the problem is solved. (maybe you need to use the fire bottle to put out the fire, but the boat is not likely to go up in flames). So a Battery switch will normally provide reasonable protection from a short...forget the big fuse. 
When away from the boat, the battery switch in the ''off'' position will isolate the battery from potential system shorts.

If you plan to use a dedicated starting battery, one with a 1000 amp cranking capacity will do a good job. I have one on my boat and it turns my 4-108 over with much more authority than the deep cycle house batteries I used before.

I don''t consider myself an expert on this but there''s my two cents worth.

Fairwinds,

Jim


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## peterpan2k (Feb 20, 2002)

I can help you if you give me a little more information.
1. How many batteries will you be using. 
2. Are you going to use this system for other electrical needs?
3. What is the distance from the battery to the starter. Including what distance there will be thru what ever isolater switch. (Perco multi position.)

For sure! You don''t use a fuse to the starter.


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## staceyneil (Nov 4, 2000)

Our brand new DC system consists of a house bank of three 8D AGMs (with it''s own seperate ON/OFF battery switch, Blue Seas rated 300 amps continuous) and a SEPERATE starting battery (as yet not sized or purchased.)

The start battery is located close to the starter motor..I would say about 36-40" or so as the wire is routed.

We have been advised to install a SLOW-BLOW fuse in this circuit. Given what other people have told me, re: starter draw, I am assuming about a 250 amp slo-blo fuse. However I have not gotten any "hard facts" yet, just various opinions.

The battery switch is an ON/OFF (also rated 250 amps continuous? Or maybe I should go up to 300 amps?) is right there in that 3-4 foot cable to the motor.

This circuit is also connected to the house circuits via an "emergency parallel" battery switch (ON/OFF), as well as a battery combiner/isolator controlled by the Ample regulator for charge regulation.

My question is what size cable I need to use (at this point I am thinking 3/0 like I will be using for the house high-amp cable and windlass), and what size battery I want (various folks have advised Group 27 with high CCA...)

Thanks for any advise!

Stacey


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## peterpan2k (Feb 20, 2002)

Cool, Ok. 250 amps is a lot for a small diesel enginge that the starter is in good shape. 175 would be more like it. I still don''t like the idea of a slow blow fuse. It''s a bit complicated but amps increase when voltage drops. The intent of a slow blow fuse is to protect a circuit from momentary high amp surge such as when an electric motor starts to turn. starter motors continue to draw at high amps when working because they are momentary use devices. Simply it won''t work as you expect. 
3/0 is a bit small to go that distance. 1/0 will better carry the load with out overheating even if the starter goes bad. Use for both pos and neg. Be sure to solder the lugs. a 27 group 750 amp hour battery will work fine. I use one each to start my olds 455''s. 
Jim


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## geohan (Mar 8, 2000)

How large is a 27 group 750 amp hour battery? I didn''t know common 12 volt batteries were available with this much capacity. Are they very expensive? Thanks, George


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## peterpan2k (Feb 20, 2002)

Hi George
I''m not standing at the local Wal-mart battery rack but i''m sure i''ve seen them there for about $49.95. they look the same as any 27. Group 24''s are usually 650 cca. There are some other automotive groups that have 850 cca but sell for more like $60.00. Only because i''ve had to deal with every type and brand over the last 35+ years. I have a little more savy about thier nature. With the exception of the very expensive gel cells, there is no differance between battery longevity, brand and type. Deep cycle''s don''t do any better at holding a charge than standard batteries. They confuse the consumer even more by rating them differently than standard batteries. They just cost more because they have have the word MARINE stamped on them. The rest is sales hype. The real indicator of a batteries ability to hold up to what ever use you''ll put it to is it''s weight. (LEAD) is what batteries are made of. The more lead, the more amps and life expectancy. It''s just that simple. i''ve had this conversation before. If you get 2 years out of what ever battery you buy you''ve been blessed. So why pay more. "Warranty" is only a sales gimmick.
Jim


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## geohan (Mar 8, 2000)

Peterpan2k: Thanks for your reply. That solves the battery mystery. I had read the ''cold cranking amperage'' for capacity in ''ampere- hours''.
Thanks, George


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## GordMay (Dec 19, 2002)

There ARE major differences between battery types and brands. For instance:
Starting Batteries have Thinner more porous plates and very large interconnecting bus bars to provide high currents for short durations.
Deep Cycle Batteries have thicker plates and lighter bus bars to provide low currents for long durations.
The Battery Council International has an excellent website at:
<www.batterycouncil.org/made.html>
Although a 1/0 cable may be adequate, LARGER is better - and in small quantities cost should not be a factor - I''d use at least 3/0 Tinned Copper (to reduce voltage drop etc.), crimped and sealed - definatly NOT soldered (solder causes a ''hard'' spot, which engine vibration breaks).
Good luck,
Gord


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## GordMay (Dec 19, 2002)

There ARE major differences between battery types and brands. For instance:
Starting Batteries have Thinner more porous plates and very large interconnecting bus bars to provide high currents for short durations.
Deep Cycle Batteries have thicker plates and lighter bus bars to provide low currents for long durations.
The Battery Council International has an excellent website at:
www.batterycouncil.org/made.html
Although a 1/0 cable may be adequate, LARGER is better - and in small quantities cost should not be a factor - I''d use at least 3/0 (which contrary to Peter''s post is LARGER than 1/0) Tinned Copper (to reduce voltage drop etc.), crimped and sealed - definatly NOT soldered (solder causes a ''hard'' spot, which engine vibration breaks).
Good luck,
Gord


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